X-Git-Url: http://matita.cs.unibo.it/gitweb/?a=blobdiff_plain;f=helm%2Fsoftware%2Fmatita%2Fnlibrary%2Ftopology%2Figft.ma;h=e6caecc6360e01dd232b3cf54328562abcf75f93;hb=ac2780dc65e14efb3fc6315825696b0b44ec230f;hp=8e05ca6f45b2e9aa1f93ef7d25b06fe5193ae64d;hpb=cde41460616634f77c2c11b585400dcc613b88f1;p=helm.git

diff --git a/helm/software/matita/nlibrary/topology/igft.ma b/helm/software/matita/nlibrary/topology/igft.ma
index 8e05ca6f4..e6caecc63 100644
--- a/helm/software/matita/nlibrary/topology/igft.ma
+++ b/helm/software/matita/nlibrary/topology/igft.ma
@@ -1,157 +1,1221 @@
+(*D
 
-include "logic/connectives.ma".
+Matita Tutorial: inductively generated formal topologies
+======================================================== 
 
-nrecord powerset (X : Type[0]) : Type[1] ≝ { char : X → CProp[0] }.
+This is a not so short introduction to Matita, based on
+the formalization of the paper
 
-interpretation "char" 'subset p = (mk_powerset ? p).  
+> Between formal topology and game theory: an
+> explicit solution for the conditions for an
+> inductive generation of formal topologies
 
-interpretation "pwset" 'powerset a = (powerset a). 
+by S.Berardi and S. Valentini. The tutorial is by Enrico Tassi. 
 
-interpretation "in" 'mem a X = (char ? X a). 
+The tutorial spends a considerable amount of effort in defining 
+notations that resemble the ones used in the original paper. We believe
+this is a important part of every formalization, not only from the aesthetic 
+point of view, but also from the practical point of view. Being 
+consistent allows to follow the paper in a pedantic way, and hopefully
+to make the formalization (at least the definitions and proved
+statements) readable to the author of the paper. 
 
-ndefinition subseteq ≝ λA.λU,V : Ω^A. 
-  ∀x.x ∈ U → x ∈ V.
+Orienteering
+------------
 
-interpretation "subseteq" 'subseteq u v = (subseteq ? u v).
+The graphical interface of Matita is composed of three windows:
+the script window, on the left, is where you type; the sequent
+window on the top right is where the system shows you the ongoing proof;
+the error window, on the bottom right, is where the system complains.
+On the top of the script window five buttons drive the processing of
+the proof script. From left to right the requesting the system to:
 
-ndefinition overlaps ≝ λA.λU,V : Ω^A. 
-  ∃x.x ∈ U ∧ x ∈ V.
+- go back to the beginning of the script
+- go back one step
+- go to the current cursor position
+- advance one step
+- advance to the end of the script
 
-interpretation "overlaps" 'overlaps u v = (overlaps ? u v).
-(*
-ndefinition intersect ≝ λA.λu,v:Ω\sup A.{ y | y ∈ u ∧ y ∈ v }.
+When the system processes a command, it locks the part of the script
+corresponding to the command, such that you cannot edit it anymore 
+(without to go back). Locked parts are coloured in blue.
 
-interpretation "intersect" 'intersects u v = (intersect ? u v). 
-*)
-nrecord axiom_set : Type[1] ≝ { 
-  S:> Type[0];
-  I: S → Type[0];
-  C: ∀a:S. I a → Ω ^ S
+The sequent window is hyper textual, i.e. you can click on symbols
+to jump to their definition, or switch between different notation
+for the same expression (for example, equality has two notations,
+one of them makes the type of the arguments explicit).  
+
+Everywhere in the script you can use the `ncheck (term).` command to
+ask for the type a given term. If you that in the middle of a proof,
+the term is assumed to live in the current proof context (i.e. can use
+variables introduced so far).
+
+To ease the typing of mathematical symbols, the script window
+implements two unusual input facilities:
+
+- some TeX symbols can be typed using their TeX names, and are 
+  automatically converted to UTF-8 characters. For a list of 
+  the supported TeX names, see the menu: View ▹ TeX/UTF-8 Table.
+  Moreover some ASCII-art is understood as well, like `=>` and `->`
+  to mean double or single arrows.
+  Here we recall some of these "shortcuts":
+
+  - ∀ can be typed with `\Forall`
+  - λ can be typed with `\lambda`
+  - ≝ can be typed with `\def` or `:=`
+  - → can be typed with `to` or `->`
+  
+- some symbols have variants, like the ≤ relation and ≼, ≰, ⋠.
+  The user can cycle between variants typing one of them and then
+  pressing ALT-L. Note that also letters do have variants, for
+  example W has Ω, 𝕎 and 𝐖, L has Λ, 𝕃, and 𝐋, F has Φ, … 
+  Variants are listed in the aforementioned TeX/UTF-8 table. 
+
+CIC (as implemented in Matita) in a nutshell
+-------------------------------------------- 
+
+CIC is a full and functional Pure Type System (all products do exist,
+and their sort is is determined by the target) with an impedicative sort
+Prop and a predicative sort Type. The environment can be populated with
+well typed definitions or theorems, (co)inductive types validating positivity
+conditions and recursive functions provably total by simple syntactical 
+analysis (recursive call on structurally smaller subterms). Co-recursive 
+functions can be defined as well, and must satisfy a dual condition, that is
+performing the recursive call only after having generated a constructor (a piece
+of output).
+
+The λ-calculus is equipped with a pattern matching construct (match) on inductive
+types defined in the environment that combined with the definable total 
+structural recursive functions allows to define eliminators (or constructors)
+for (co)inductive types. The λ-calculus is also equipped with explicitly types 
+local definitions (let in) that in the degenerate case work as casts (i.e.
+the type annotation `(t : T)`  is implemented as `let x : T ≝ t in x`). 
+
+Types are compare up to conversion. Since types may depend on terms, conversion
+involves β-reduction, δ-reduction (definition unfolding), ζ-reduction (local
+definition unfolding), ι-reduction (pattern matching simplification),
+μ-reduction (recursive function computation) and ν-reduction (co-fixpoint
+computation).
+
+Since we are going to formalize constructive and predicative mathematics
+in an intensional type theory like CIC, we try to establish some terminology. 
+Type is the sort of sets equipped with the Id equality (i.e. an intensional,
+not quotiented set). We will avoid using Leibnitz equality Id, 
+thus we will explicitly equip a set with an equality when needed.
+We will call this structure `setoid`. Note that we will
+attach the infix = symbol only to the equality of a setoid,
+not to Id.
+
+We write Type[i] to mention a Type in the predicative hierarchy 
+of types. To ease the comprehension we will use Type[0] for sets, 
+and Type[1] for classes.
+
+For every Type[i] there is a corresponding level of predicative
+propositions CProp[i].
+
+
+The standard library and the `include` command
+----------------------------------------------
+
+Some basic notions, like subset, membership, intersection and union
+are part of the standard library of Matita.
+
+These notions come with some standard notation attached to them:
+
+- A ∪ B `A \cup B`
+- A ∩ B `A \cap B` 
+- A ≬ B `A \between B`
+- x ∈ A `x \in A` 
+- Ω^A, that is the type of the subsets of A, `\Omega ^ A` 
+
+The `include` command tells Matita to load a part of the library, 
+in particular the part that we will use can be loaded as follows: 
+
+D*)
+
+include "sets/sets.ma".
+
+(*D
+
+Some basic results that we will use are also part of the sets library:
+
+- subseteq\_union\_l: ∀A.∀U,V,W:Ω^A.U ⊆ W → V ⊆ W → U ∪ V ⊆ W
+- subseteq\_intersection\_r: ∀A.∀U,V,W:Ω^A.W ⊆ U → W ⊆ V → W ⊆ U ∩ V
+
+Defining Axiom set
+------------------
+
+A set of axioms is made of a set S, a family of sets I and a 
+family C of subsets of S indexed by elements a of S and I(a).
+
+It is desirable to state theorems like "for every set of axioms, …"
+without explicitly mentioning S, I and C. To do that, the three 
+components have to be grouped into a record (essentially a dependently
+typed tuple). The system is able to generate the projections
+of the record for free, and they are named as the fields of
+the record. So, given a axiom set `A` we can obtain the set
+with `S A`, the family of sets with `I A` and the family of subsets
+with `C A`.
+
+D*)
+
+nrecord Ax : Type[1] ≝ { 
+  S :> setoid;
+  I :  S → Type[0];
+  C :  ∀a:S. I a → Ω ^ S
 }.
 
-ndefinition cover_set ≝ λc:∀A:axiom_set.Ω^A → A → CProp[0].λA,C,U.
-  ∀y.y ∈ C → c A U y.
+(*D
+
+Forget for a moment the `:>` that will be detailed later, and focus on
+the record definition. It is made of a list of pairs: a name, followed
+by `:` and the its type. It is a dependently typed tuple, thus
+already defined names (fields) can be used in the types that follow. 
+
+Note that `S` is declared to be a `setoid` and not a Type. The original
+paper probably also considers I to generate setoids, and both I and C
+to be morphisms. For the sake of simplicity, we will "cheat" and use
+setoids only when strictly needed (i.e. where we want to talk about 
+equality). Setoids will play a role only when we will define
+the alternative version of the axiom set.
+
+Note that the field `S` was declared with `:>` instead of a simple `:`.
+This declares the `S` projection to be a coercion. A coercion is 
+a function case the system automatically inserts when it is needed.
+In that case, the projection `S` has type `Ax → setoid`, and whenever
+the expected type of a term is `setoid` while its type is `Ax`, the 
+system inserts the coercion around it, to make the whole term well types.
+
+When formalizing an algebraic structure, declaring the carrier as a 
+coercion is a common practice, since it allows to write statements like
+
+    ∀G:Group.∀x:G.x * x^-1 = 1 
+
+The quantification over `x` of type `G` is ill-typed, since `G` is a term
+(of type `Group`) and thus not a type. Since the carrier projection 
+`carr` of `G` is a coercion, that maps a `Group` into the type of 
+its elements, the system automatically inserts `carr` around `G`, 
+obtaining `…∀x: carr G.…`. Coercions are also hidden by the system
+when it displays a term.
+
+In this particular case, the coercion `S` allows to write
+
+    ∀A:Ax.∀a:A.…
+
+Since `A` is not a type, but it can be turned into a `setoid` by `S`
+and a `setoid` can be turned into a type by its `carr` projection, the 
+composed coercion `carr ∘ S` is silently inserted.
+
+Implicit arguments
+------------------
+
+Something that is not still satisfactory, is that the dependent type
+of `I` and `C` are abstracted over the Axiom set. To obtain the
+precise type of a term, you can use the `ncheck` command as follows.
+
+D*) 
+
+(* ncheck I. *)
+(* ncheck C. *)
+
+(*D
+
+One would like to write `I a` and not `I A a` under a context where
+`A` is an axiom set and `a` has type `S A` (or thanks to the coercion
+mechanism simply `A`). In Matita, a question mark represents an implicit
+argument, i.e. a missing piece of information the system is asked to
+infer. Matita performs some sort of type inference, thus writing
+`I ? a` is enough: since the second argument of `I` is typed by the 
+first one, the first one can be inferred just computing the type of `a`.
+
+D*) 
+
+(* ncheck (∀A:Ax.∀a:A.I ? a). *)
+
+(*D
+
+This is still not completely satisfactory, since you have always type 
+`?`; to fix this minor issue we have to introduce the notational
+support built in Matita.
+
+Notation for I and C
+--------------------
+
+Matita is quipped with a quite complex notational support,
+allowing the user to define and use mathematical notations 
+([From Notation to Semantics: There and Back Again][1]). 
+
+Since notations are usually ambiguous (e.g. the frequent overloading of 
+symbols) Matita distinguishes between the term level, the 
+content level, and the presentation level, allowing multiple 
+mappings between the content and the term level. 
+
+The mapping between the presentation level (i.e. what is typed on the 
+keyboard and what is displayed in the sequent window) and the content
+level is defined with the `notation` command. When followed by
+`>`, it defines an input (only) notation.   
+
+D*)
+
+notation > "𝐈 term 90 a" non associative with precedence 70 for @{ 'I $a }.
+notation > "𝐂 term 90 a term 90 i" non associative with precedence 70 for @{ 'C $a $i }.
+
+(*D
+
+The first notation defines the writing `𝐈 a` where `a` is a generic
+term of precedence 90, the maximum one. This high precedence forces
+parentheses around any term of a lower precedence. For example `𝐈 x`
+would be accepted, since identifiers have precedence 90, but
+`𝐈 f x` would be interpreted as `(𝐈 f) x`. In the latter case, parentheses
+have to be put around `f x`, thus the accepted writing would be `𝐈 (f x)`.
+
+To obtain the `𝐈` is enough to type `I` and then cycle between its
+similar symbols with ALT-L. The same for `𝐂`. Notations cannot use
+regular letters or the round parentheses, thus their variants (like the 
+bold ones) have to be used.
+
+The first notation associates `𝐈 a` with `'I $a` where `'I` is a 
+new content element to which a term `$a` is passed.
+
+Content elements have to be interpreted, and possibly multiple, 
+incompatible, interpretations can be defined.
+
+D*)
+
+interpretation "I" 'I a = (I ? a).
+interpretation "C" 'C a i = (C ? a i).
+
+(*D
+
+The `interpretation` command allows to define the mapping between
+the content level and the terms level. Here we associate the `I` and
+`C` projections of the Axiom set record, where the Axiom set is an implicit 
+argument `?` to be inferred by the system.
+
+Interpretation are bi-directional, thus when displaying a term like 
+`C _ a i`, the system looks for a presentation for the content element
+`'C a i`. 
+
+D*)
+
+notation < "𝐈  \sub( ❨a❩ )" non associative with precedence 70 for @{ 'I $a }.
+notation < "𝐂 \sub( ❨a,\emsp i❩ )" non associative with precedence 70 for @{ 'C $a $i }.
+
+(*D
+
+For output purposes we can define more complex notations, for example
+we can put bold parentheses around the arguments of `𝐈` and `𝐂`, decreasing
+the size of the arguments and lowering their baseline (i.e. putting them
+as subscript), separating them with a comma followed by a little space.
+
+The first (technical) definition
+--------------------------------
+
+Before defining the cover relation as an inductive predicate, one
+has to notice that the infinity rule uses, in its hypotheses, the 
+cover relation between two subsets, while the inductive predicate 
+we are going to define relates an element and a subset.
+
+An option would be to unfold the definition of cover between subsets,
+but we prefer to define the abstract notion of cover between subsets
+(so that we can attach a (ambiguous) notation to it).
+
+Anyway, to ease the understanding of the definition of the cover relation 
+between subsets, we first define the inductive predicate unfolding the 
+definition, and we later refine it with.
+
+D*)
+
+ninductive xcover (A : Ax) (U : Ω^A) : A → CProp[0] ≝ 
+| xcreflexivity : ∀a:A. a ∈ U → xcover A U a
+| xcinfinity    : ∀a:A.∀i:𝐈 a. (∀y.y ∈ 𝐂 a i → xcover A U y) → xcover A U a.
+
+(*D
+
+We defined the xcover (x will be removed in the final version of the 
+definition) as an inductive predicate. The arity of the inductive
+predicate has to be carefully analyzed:
+
+>  (A : Ax) (U : Ω^A) : A → CProp[0]
+
+The syntax separates with `:` abstractions that are fixed for every
+constructor (introduction rule) and abstractions that can change. In that 
+case the parameter `U` is abstracted once and forall in front of every 
+constructor, and every occurrence of the inductive predicate is applied to
+`U` in a consistent way. Arguments abstracted on the right of `:` are not
+constant, for example the xcinfinity constructor introduces `a ◃ U`,
+but under the assumption that (for every y) `y ◃ U`. In that rule, the left
+had side of the predicate changes, thus it has to be abstracted (in the arity
+of the inductive predicate) on the right of `:`.
+
+D*)
+
+(* ncheck xcreflexivity. *)
+
+(*D
+
+We want now to abstract out `(∀y.y ∈ 𝐂 a i → xcover A U y)` and define
+a notion `cover_set` to which a notation `𝐂 a i ◃ U` can be attached.
+
+This notion has to be abstracted over the cover relation (whose
+type is the arity of the inductive `xcover` predicate just defined).
+
+Then it has to be abstracted over the arguments of that cover relation,
+i.e. the axiom set and the set U, and the subset (in that case `𝐂 a i`)
+sitting on the left hand side of `◃`. 
+
+D*)
+
+ndefinition cover_set : 
+  ∀cover: ∀A:Ax.Ω^A → A → CProp[0]. ∀A:Ax.∀C,U:Ω^A. CProp[0] 
+≝ 
+  λcover.                           λA,    C,U.     ∀y.y ∈ C → cover A U y.
+
+(*D
+
+The `ndefinition` command takes a name, a type and body (of that type).
+The type can be omitted, and in that case it is inferred by the system.
+If the type is given, the system uses it to infer implicit arguments
+of the body. In that case all types are left implicit in the body.
+
+We now define the notation `a ◃ b`. Here the keywork `hvbox`
+and `break` tell the system how to wrap text when it does not
+fit the screen (they can be safely ignore for the scope of
+this tutorial). We also add an interpretation for that notation, 
+where the (abstracted) cover relation is implicit. The system
+will not be able to infer it from the other arguments `C` and `U`
+and will thus prompt the user for it. This is also why we named this 
+interpretation `covers set temp`: we will later define another 
+interpretation in which the cover relation is the one we are going to 
+define.
+
+D*)
 
 notation "hvbox(a break ◃ b)" non associative with precedence 45
-for @{ 'covers $a $b }. (* a \ltri b *)
+for @{ 'covers $a $b }.
 
 interpretation "covers set temp" 'covers C U = (cover_set ?? C U).
 
-ninductive cover (A : axiom_set) (U : Ω^A) : A → CProp[0] ≝ 
-| creflexivity : ∀a:A. a ∈ U → cover ? U a
-| cinfinity    : ∀a:A. ∀i:I ? a. (C ? a i ◃ U) → cover ? U a.
+(*D
+
+The cover relation
+------------------
+
+We can now define the cover relation using the `◃` notation for 
+the premise of infinity. 
+
+D*)
+
+ninductive cover (A : Ax) (U : Ω^A) : A → CProp[0] ≝ 
+| creflexivity : ∀a. a ∈ U → cover ? U a
+| cinfinity    : ∀a. ∀i. 𝐂 a i ◃ U → cover ? U a.
+(** screenshot "cover". *) 
 napply cover;
 nqed.
 
+(*D
+
+Note that the system accepts the definition
+but prompts the user for the relation the `cover_set` notion is
+abstracted on.
+
+
+
+The horizontal line separates the hypotheses from the conclusion.
+The `napply cover` command tells the system that the relation
+it is looking for is exactly our first context entry (i.e. the inductive
+predicate we are defining, up to α-conversion); while the `nqed` command
+ends a definition or proof.
+
+We can now define the interpretation for the cover relation between an
+element and a subset fist, then between two subsets (but this time
+we fixed the relation `cover_set` is abstracted on).
+
+D*)
+
 interpretation "covers" 'covers a U = (cover ? U a).
 interpretation "covers set" 'covers a U = (cover_set cover ? a U).
 
-ndefinition fish_set ≝ λf:∀A:axiom_set.Ω^A → A → CProp[0].
+(*D
+
+We will proceed similarly for the fish relation, but before going
+on it is better to give a short introduction to the proof mode of Matita.
+We define again the `cover_set` term, but this time we will build
+its body interactively. In the λ-calculus Matita is based on, CIC, proofs
+and terms share the same syntax, and it is thus possible to use the
+commands devoted to build proof term to build regular definitions.
+A tentative semantics for the proof mode commands (called tactics)
+in terms of sequent calculus rules are given in the
+<a href="#appendix">appendix</a>.
+
+D*)
+
+ndefinition xcover_set : 
+  ∀c: ∀A:Ax.Ω^A → A → CProp[0]. ∀A:Ax.∀C,U:Ω^A. CProp[0]. 
+                         (** screenshot "xcover-set-1". *)
+#cover; #A; #C; #U;      (** screenshot "xcover-set-2". *) 
+napply (∀y:A.y ∈ C → ?); (** screenshot "xcover-set-3". *)
+napply cover;            (** screenshot "xcover-set-4". *)
+##[ napply A;
+##| napply U;
+##| napply y;
+##]
+nqed.
+
+(*D[xcover-set-1]
+The system asks for a proof of the full statement, in an empty context.
+
+The `#` command is the ∀-introduction rule, it gives a name to an 
+assumption putting it in the context, and generates a λ-abstraction
+in the proof term.
+
+D[xcover-set-2]
+We have now to provide a proposition, and we exhibit it. We left
+a part of it implicit; since the system cannot infer it it will
+ask it later. Note that the type of `∀y:A.y ∈ C → ?` is a proposition
+whenever `?` is.
+
+D[xcover-set-3]
+The proposition we want to provide is an application of the
+cover relation we have abstracted in the context. The command
+`napply`, if the given term has not the expected type (in that
+case it is a product versus a proposition) it applies it to as many 
+implicit arguments as necessary (in that case `? ? ?`).
+
+D[xcover-set-4]
+The system will now ask in turn the three implicit arguments 
+passed to cover. The syntax `##[` allows to start a branching
+to tackle every sub proof individually, otherwise every command
+is applied to every subrpoof. The command `##|` switches to the next
+subproof and `##]` ends the branching.  
+D*)
+
+(*D
+
+The fish relation
+-----------------
+
+The definition of fish works exactly the same way as for cover, except 
+that it is defined as a coinductive proposition.
+D*)
+
+ndefinition fish_set ≝ λf:∀A:Ax.Ω^A → A → CProp[0].
  λA,U,V.
   ∃a.a ∈ V ∧ f A U a.
 
+(* a \ltimes b *)
 notation "hvbox(a break ⋉ b)" non associative with precedence 45
-for @{ 'fish $a $b }. (* a \ltimes b *)
+for @{ 'fish $a $b }. 
 
 interpretation "fish set temp" 'fish A U = (fish_set ?? U A).
 
-ncoinductive fish (A : axiom_set) (F : Ω^A) : A → CProp[0] ≝ 
-| cfish : ∀a. a ∈ F → (∀i:I ? a.C A a i ⋉ F) → fish A F a.
+ncoinductive fish (A : Ax) (F : Ω^A) : A → CProp[0] ≝ 
+| cfish : ∀a. a ∈ F → (∀i:𝐈 a .𝐂  a i ⋉ F) → fish A F a.
 napply fish;
 nqed.
 
 interpretation "fish set" 'fish A U = (fish_set fish ? U A).
 interpretation "fish" 'fish a U = (fish ? U a).
 
-nlet corec fish_rec (A:axiom_set) (U: Ω^A)
+(*D
+
+Introction rule for fish
+------------------------
+
+Matita is able to generate elimination rules for inductive types,
+but not introduction rules for the coinductive case. 
+
+D*)
+
+(* ncheck cover_rect_CProp0. *) 
+
+(*D
+
+We thus have to define the introduction rule for fish by corecursion.
+Here we again use the proof mode of Matita to exhibit the body of the
+corecursive function.
+
+D*)
+
+nlet corec fish_rec (A:Ax) (U: Ω^A)
  (P: Ω^A) (H1: P ⊆ U)
-  (H2: ∀a:A. a ∈ P → ∀j: I ? a. C ? a j ≬ P):
-   ∀a:A. ∀p: a ∈ P. a ⋉ U ≝ ?.
-#a; #p; napply cfish;
-##[ napply H1; napply p;
-##| #i; ncases (H2 a p i); #x; *; #xC; #xP; napply ex_intro; ##[napply x]
-    napply conj; ##[ napply xC ] napply (fish_rec ? U P); nassumption;
+  (H2: ∀a:A. a ∈ P → ∀j: 𝐈 a. 𝐂 a j ≬ P): ∀a:A. ∀p: a ∈ P. a ⋉ U ≝ ?.
+                                       (** screenshot "def-fish-rec-1".   *)
+#a; #p; napply cfish;                  (** screenshot "def-fish-rec-2".   *)
+##[ nchange in H1 with (∀b.b∈P → b∈U); (** screenshot "def-fish-rec-2-1". *) 
+    napply H1;                         (** screenshot "def-fish-rec-3".   *) 
+    nassumption;
+##| #i; ncases (H2 a p i);             (** screenshot "def-fish-rec-5".   *) 
+    #x; *; #xC; #xP;                   (** screenshot "def-fish-rec-5-1". *) 
+    @;                                 (** screenshot "def-fish-rec-6".   *)
+    ##[ napply x
+    ##| @;                             (** screenshot "def-fish-rec-7".   *)
+        ##[ napply xC; 
+        ##| napply (fish_rec ? U P);   (** screenshot "def-fish-rec-9".   *)
+            nassumption;
+        ##]
+    ##]
+##]
+nqed.
+
+(*D
+D[def-fish-rec-1]
+Note the first item of the context, it is the corecursive function we are 
+defining. This item allows to perform the recursive call, but we will be
+allowed to do such call only after having generated a constructor of
+the fish coinductive type.
+
+We introduce `a` and `p`, and then return the fish constructor `cfish`.
+Since the constructor accepts two arguments, the system asks for them.
+
+D[def-fish-rec-2]
+The first one is a proof that `a ∈ U`. This can be proved using `H1` and `p`.
+With the `nchange` tactic we change `H1` into an equivalent form (this step
+can be skipped, since the system would be able to unfold the definition
+of inclusion by itself)
+
+D[def-fish-rec-2-1]
+It is now clear that `H1` can be applied. Again `napply` adds two 
+implicit arguments to `H1 ? ?`, obtaining a proof of `? ∈ U` given a proof
+that `? ∈ P`. Thanks to unification, the system understands that `?` is actually
+`a`, and it asks a proof that `a ∈ P`.
+
+D[def-fish-rec-3]
+The `nassumption` tactic looks for the required proof in the context, and in
+that cases finds it in the last context position. 
+
+We move now to the second branch of the proof, corresponding to the second
+argument of the `cfish` constructor.
+
+We introduce `i` and then we destruct `H2 a p i`, that being a proof
+of an overlap predicate, give as an element and a proof that it is 
+both in `𝐂 a i` and `P`.
+
+D[def-fish-rec-5]
+We then introduce `x`, break the conjunction (the `*;` command is the
+equivalent of `ncases` but operates on the first hypothesis that can
+be introduced. We then introduce the two sides of the conjunction.
+
+D[def-fish-rec-5-1]
+The goal is now the existence of an a point in `𝐂 a i` fished by `U`.
+We thus need to use the introduction rule for the existential quantifier.
+In CIC it is a defined notion, that is an inductive type with just
+one constructor (one introduction rule) holding the witness and the proof
+that the witness satisfies a proposition.
+
+> ncheck Ex.
+
+Instead of trying to remember the name of the constructor, that should
+be used as the argument of `napply`, we can ask the system to find by
+itself the constructor name and apply it with the `@` tactic. 
+Note that some inductive predicates, like the disjunction, have multiple
+introduction rules, and thus `@` can be followed by a number identifying
+the constructor.
+
+D[def-fish-rec-6]
+After choosing `x` as the witness, we have to prove a conjunction,
+and we again apply the introduction rule for the inductively defined
+predicate `∧`.
+
+D[def-fish-rec-7]
+The left hand side of the conjunction is trivial to prove, since it 
+is already in the context. The right hand side needs to perform
+the co-recursive call.
+
+D[def-fish-rec-9]
+The co-recursive call needs some arguments, but all of them live
+in the context. Instead of explicitly mention them, we use the
+`nassumption` tactic, that simply tries to apply every context item.
+
+D*)
+
+(*D
+
+Subset of covered/fished points
+-------------------------------
+
+We now have to define the subset of `S` of points covered by `U`.
+We also define a prefix notation for it. Remember that the precedence
+of the prefix form of a symbol has to be lower than the precedence 
+of its infix form.
+
+D*)
+
+ndefinition coverage : ∀A:Ax.∀U:Ω^A.Ω^A ≝ λA,U.{ a | a ◃ U }.
+
+notation "◃U" non associative with precedence 55 for @{ 'coverage $U }.
+
+interpretation "coverage cover" 'coverage U = (coverage ? U).
+
+(*D
+
+Here we define the equation characterizing the cover relation. 
+In the igft.ma file we proved that `◃U` is the minimum solution for
+such equation, the interested reader should be able to reply the proof
+with Matita.
+
+D*)
+
+ndefinition cover_equation : ∀A:Ax.∀U,X:Ω^A.CProp[0] ≝  λA,U,X. 
+  ∀a.a ∈ X ↔ (a ∈ U ∨ ∃i:𝐈 a.∀y.y ∈ 𝐂 a i → y ∈ X).  
+
+ntheorem coverage_cover_equation : ∀A,U. cover_equation A U (◃U).
+#A; #U; #a; @; #H;
+##[ nelim H; #b; 
+    ##[ #bU; @1; nassumption;
+    ##| #i; #CaiU; #IH; @2; @ i; #c; #cCbi; ncases (IH ? cCbi);
+        ##[ #E; @; napply E;
+        ##| #_; napply CaiU; nassumption; ##] ##]
+##| ncases H; ##[ #E; @; nassumption]
+    *; #j; #Hj; @2 j; #w; #wC; napply Hj; nassumption;
+##]
+nqed. 
+
+ntheorem coverage_min_cover_equation : 
+  ∀A,U,W. cover_equation A U W → ◃U ⊆ W.
+#A; #U; #W; #H; #a; #aU; nelim aU; #b;
+##[ #bU; ncases (H b); #_; #H1; napply H1; @1; nassumption;
+##| #i; #CbiU; #IH; ncases (H b); #_; #H1; napply H1; @2; @i; napply IH;
 ##]
 nqed.
 
-alias symbol "covers" = "covers".
-alias symbol "covers" = "covers".
-ntheorem covers_elim2:
- ∀A: axiom_set. ∀U:Ω^A.∀P: A → CProp[0].
-  (∀a:A. a ∈ U → P a) →
-   (∀a:A.∀V:Ω^A. a ◃ V → V ◃ U → (∀y. y ∈ V → P y) → P a) →
-     ∀a:A. a ◃ U → P a.
-#A; #U; #P; #H1; #H2; #a; #aU; nelim aU;
-##[ #b; #H; napply H1; napply H;
-##| #b; #i; #CaiU; #H; napply H2; 
-    ##[ napply (C ? b i);
-    ##| napply cinfinity; ##[ napply i ] nwhd; #y; #H3; napply creflexivity; ##]
-    nassumption; 
+(*D
+
+We similarly define the subset of point fished by `F`, the 
+equation characterizing `⋉F` and prove that fish is
+the biggest solution for such equation.
+
+D*) 
+
+notation "⋉F" non associative with precedence 55
+for @{ 'fished $F }.
+
+ndefinition fished : ∀A:Ax.∀F:Ω^A.Ω^A ≝ λA,F.{ a | a ⋉ F }.
+
+interpretation "fished fish" 'fished F = (fished ? F).
+
+ndefinition fish_equation : ∀A:Ax.∀F,X:Ω^A.CProp[0] ≝ λA,F,X.
+  ∀a. a ∈ X ↔ a ∈ F ∧ ∀i:𝐈 a.∃y.y ∈ 𝐂 a i ∧ y ∈ X. 
+  
+ntheorem fished_fish_equation : ∀A,F. fish_equation A F (⋉F).
+#A; #F; #a; @; (* *; non genera outtype che lega a *) #H; ncases H;
+##[ #b; #bF; #H2; @ bF; #i; ncases (H2 i); #c; *; #cC; #cF; @c; @ cC;
+    napply cF;  
+##| #aF; #H1; @ aF; napply H1;
 ##]
 nqed.
 
-STOP
+ntheorem fished_max_fish_equation : ∀A,F,G. fish_equation A F G → G ⊆ ⋉F.
+#A; #F; #G; #H; #a; #aG; napply (fish_rec … aG);
+#b; ncases (H b); #H1; #_; #bG; ncases (H1 bG); #E1; #E2; nassumption; 
+nqed. 
+
+(*D
+
+Part 2, the new set of axioms
+-----------------------------
+
+Since the name of objects (record included) has to unique
+within the same script, we prefix every field name
+in the new definition of the axiom set with `n`.
+
+D*)
+
+nrecord nAx : Type[2] ≝ { 
+  nS:> setoid; 
+  nI: nS → Type[0];
+  nD: ∀a:nS. nI a → Type[0];
+  nd: ∀a:nS. ∀i:nI a. nD a i → nS
+}.
+
+(*D
+
+We again define a notation for the projections, making the 
+projected record an implicit argument. Note that since we already have
+a notation for `𝐈` we just add another interpretation for it. The
+system, looking at the argument of `𝐈`, will be able to use
+the correct interpretation. 
+
+D*)
+
+notation "𝐃 \sub ( ❨a,\emsp i❩ )" non associative with precedence 70 for @{ 'D $a $i }.
+notation "𝐝 \sub ( ❨a,\emsp i,\emsp j❩ )" non associative with precedence 70 for @{ 'd $a $i $j}.
+
+notation > "𝐃 term 90 a term 90 i" non associative with precedence 70 for @{ 'D $a $i }.
+notation > "𝐝 term 90 a term 90 i term 90 j" non associative with precedence 70 for @{ 'd $a $i $j}.
+
+interpretation "D" 'D a i = (nD ? a i).
+interpretation "d" 'd a i j = (nd ? a i j).
+interpretation "new I" 'I a = (nI ? a).
+
+(*D
+
+The paper defines the image as
+
+> Im[d(a,i)] = { d(a,i,j) | j : D(a,i) }
+
+but this not so formal notation poses some problems. The image is
+often used as the left hand side of the ⊆ predicate
+
+> Im[d(a,i)] ⊆ V
+
+Of course this writing is interpreted by the authors as follows 
+
+> ∀j:D(a,i). d(a,i,j) ∈ V
+
+If we need to use the image to define 𝐂 (a subset of S) we are obliged to
+form a subset, i.e. to place a single variable `{ here | … }`.
+
+> Im[d(a,i)] = { y | ∃j:D(a,i). y = d(a,i,j) }
+
+This poses no theoretical problems, since `S` is a setoid and thus equipped
+with an equality.
+
+Unless we difene two different images, one for stating the image is ⊆ of
+something and another one to define 𝐂, we end up using always the latter.
+Thus the statement `Im[d(a,i)] ⊆ V` unfolds to
+
+> ∀x:S. ( ∃j.x = d(a,i,j) ) → x ∈ V
+
+That up to rewriting with the equation defining `x` is what we mean. The 
+technical problem arises when `V` is a complex construction that has to
+be proved extensional (i.e. ∀x,y. x = y → x ∈ V → y ∈ V).
+
+D*)
+
+ndefinition image ≝ λA:nAx.λa:A.λi. { x | ∃j:𝐃 a i. x = 𝐝 a i j }.
+
+notation > "𝐈𝐦  [𝐝 term 90 a term 90 i]" non associative with precedence 70 for @{ 'Im $a $i }.
+notation "𝐈𝐦  [𝐝 \sub ( ❨a,\emsp i❩ )]" non associative with precedence 70 for @{ 'Im $a $i }.
+
+interpretation "image" 'Im a i = (image ? a i).
+
+(*D
+
+Thanks to our definition of image, we ca trivially a function mapping a
+new axiom set to an old one and viceversa. Note that in the second
+definition, when defining 𝐝, the projection of the Σ type is inlined
+(constructed on the fly by `*;`) while in the paper it was named `fst`.
+
+D*)
+
+ndefinition Ax_of_nAx : nAx → Ax.
+#A; @ A (nI ?); #a; #i; napply (𝐈𝐦 [𝐝 a i]);
+nqed.
+
+ndefinition nAx_of_Ax : Ax → nAx.
+#A; @ A (I ?);
+##[ #a; #i; napply (Σx:A.x ∈ 𝐂 a i);
+##| #a; #i; *; #x; #_; napply x;
+##]
+nqed.
+
+(*D
+
+We then define the inductive type of ordinals, parametrized over an axiom
+set. We also attach some notations to the constructors.
+
+D*)
+
+ninductive Ord (A : nAx) : Type[0] ≝ 
+ | oO : Ord A
+ | oS : Ord A → Ord A
+ | oL : ∀a:A.∀i.∀f:𝐃 a i → Ord A. Ord A.
+
+notation "0" non associative with precedence 90 for @{ 'oO }.
+notation "Λ term 90 f" non associative with precedence 50 for @{ 'oL $f }.
+notation "x+1" non associative with precedence 50 for @{'oS $x }.
+
+interpretation "ordinals Zero" 'oO = (oO ?).
+interpretation "ordinals Lambda" 'oL f = (oL ? ? ? f).
+interpretation "ordinals Succ" 'oS x = (oS ? x).
+
+(*D
+
+The dfinition of `U_x` is by recursion over the ordinal `x`. 
+We thus define a recursive function. The `on x` directive tells
+the system on which argument the function is (structurally) recursive.
+
+In the `oS` case we use a local definition to name the recursive call
+since it is used twice.
+
+Note that Matita does not support notation in the left hand side
+of a pattern match, and thus the names of the constructors have to 
+be spelled out verbatim.
+
+D*)
+
+nlet rec famU (A : nAx) (U : Ω^A) (x : Ord A) on x : Ω^A ≝ 
+  match x with
+  [ oO ⇒ U
+  | oS y ⇒ let U_n ≝ famU A U y in U_n ∪ { x | ∃i.𝐈𝐦[𝐝 x i] ⊆ U_n} 
+  | oL a i f ⇒ { x | ∃j.x ∈ famU A U (f j) } ].
+  
+notation < "term 90 U \sub (term 90 x)" non associative with precedence 50 for @{ 'famU $U $x }.
+notation > "U ⎽ term 90 x" non associative with precedence 50 for @{ 'famU $U $x }.
+
+interpretation "famU" 'famU U x = (famU ? U x).
+
+(*D
+
+We attach as the input notation for U_x the similar `U⎽x` where underscore,
+that is a character valid for identifier names, has been replaced by `⎽` that is
+not. The symbol `⎽` can act as a separator, and can be typed as an alternative
+for `_` (i.e. pressing ALT-L after `_`). 
+
+The notion ◃(U) has to be defined as the subset of of y 
+belonging to U_x for some x. Moreover, we have to define the notion
+of cover between sets again, since the one defined at the beginning
+of the tutorial works only for the old axiom set definition.
+
+D*)
+  
+ndefinition ord_coverage : ∀A:nAx.∀U:Ω^A.Ω^A ≝ λA,U.{ y | ∃x:Ord A. y ∈ famU ? U x }.
+
+ndefinition ord_cover_set ≝ λc:∀A:nAx.Ω^A → Ω^A.λA,C,U.
+  ∀y.y ∈ C → y ∈ c A U.
+
+interpretation "coverage new cover" 'coverage U = (ord_coverage ? U).
+interpretation "new covers set" 'covers a U = (ord_cover_set ord_coverage ? a U).
+interpretation "new covers" 'covers a U = (mem ? (ord_coverage ? U) a).
+
+(*D
+
+Before proving that this cover relation validates the reflexivity and infinity
+rules, we prove this little technical lemma that is used in the proof for the 
+infinity rule.
+
+D*)
+
+nlemma ord_subset: ∀A:nAx.∀a:A.∀i,f,U.∀j:𝐃 a i. U⎽(f j) ⊆ U⎽(Λ f).
+#A; #a; #i; #f; #U; #j; #b; #bUf; @ j; nassumption;
+nqed.
+
+(*D
+
+The proof of infinity uses the following form of the Axiom of choice,
+that cannot be prove inside Matita, since the existential quantifier
+lives in the sort of predicative propositions while the sigma in the conclusion
+lives in the sort of data types, and thus the former cannot be eliminated
+to provide the second.
+
+D*)
+
+naxiom AC : ∀A,a,i,U.
+  (∀j:𝐃 a i.∃x:Ord A.𝐝 a i j ∈ U⎽x) → (Σf.∀j:𝐃 a i.𝐝 a i j ∈ U⎽(f j)).
+
+(*D
+
+In the proof of infinity, we have to rewrite under the ∈ predicate.
+It is clearly possible to show that U_x is an extensional set:
+
+> a = b → a ∈ U_x → b ∈ U_x
+
+Anyway this proof in non trivial induction over x, that requires 𝐈 and 𝐃 to be
+declared as morphisms. This poses to problem, but goes out of the scope of the 
+tutorial and we thus assume it.
+
+D*)
+
+naxiom setoidification: ∀A:nAx.∀a,b:A.∀x.∀U. a = b → b ∈ U⎽x → a ∈ U⎽x.
+
+(*D
+
+The reflexivity proof is trivial, it is enough to provide the ordinal 0
+as a witness, then ◃(U) reduces to U by definition, hence the conclusion.
+
+D*)
+ntheorem new_coverage_reflexive: ∀A:nAx.∀U:Ω^A.∀a. a ∈ U → a ◃ U.
+#A; #U; #a; #H; @ (0); napply H;
+nqed.
+
+(*D
+
+We now proceed with the proof of the infinity rule.
+
+D*)
+
+alias symbol "covers" = "new covers set".
+alias symbol "covers" = "new covers".
+alias symbol "covers" = "new covers set".
+alias symbol "covers" = "new covers".
+alias symbol "covers" = "new covers set".
+alias symbol "covers" = "new covers".
+alias symbol "covers" = "new covers set".
+ntheorem new_coverage_infinity:
+  ∀A:nAx.∀U:Ω^A.∀a:A. (∃i:𝐈 a. 𝐈𝐦[𝐝 a i] ◃ U) → a ◃ U.
+#A; #U; #a;                                   (** screenshot "n-cov-inf-1". *)  
+*; #i; #H; nnormalize in H;                   (** screenshot "n-cov-inf-2". *)
+ncut (∀y:𝐃 a i.∃x:Ord A.𝐝 a i y ∈ U⎽x); ##[    (** screenshot "n-cov-inf-3". *)
+  #z; napply H; @ z; napply #; ##] #H';       (** screenshot "n-cov-inf-4". *)
+ncases (AC … H'); #f; #Hf;                    (** screenshot "n-cov-inf-5". *)
+ncut (∀j.𝐝 a i j ∈ U⎽(Λ f));
+  ##[ #j; napply (ord_subset … f … (Hf j));##] #Hf';(** screenshot "n-cov-inf-6". *)
+@ (Λ f+1);                                    (** screenshot "n-cov-inf-7". *)
+@2;                                           (** screenshot "n-cov-inf-8". *) 
+@i; #x; *; #d; #Hd;                           (** screenshot "n-cov-inf-9". *)  
+napply (setoidification … Hd); napply Hf';
+nqed.
+
+(*D
+D[n-cov-inf-1]
+We eliminate the existential, obtaining an `i` and a proof that the 
+image of d(a,i) is covered by U. The `nnormalize` tactic computes the normal
+form of `H`, thus expands the definition of cover between sets.
+
+D[n-cov-inf-2]
+The paper proof considers `H` implicitly substitutes the equation assumed
+by `H` in its conclusion. In Matita this step is not completely trivia.
+We thus assert (`ncut`) the nicer form of `H`.
+
+D[n-cov-inf-3]
+After introducing `z`, `H` can be applied (choosing `𝐝 a i z` as `y`). 
+What is the left to prove is that `∃j: 𝐃 a j. 𝐝 a i z = 𝐝 a i j`, that 
+becomes trivial is `j` is chosen to be `z`. In the command `napply #`,
+the `#` is a standard notation for the reflexivity property of the equality. 
+
+D[n-cov-inf-4]
+Under `H'` the axiom of choice `AC` can be eliminated, obtaining the `f` and 
+its property.
+
+D[n-cov-inf-5]
+The paper proof does now a forward reasoning step, deriving (by the ord_subset 
+lemma we proved above) `Hf'` i.e. 𝐝 a i j ∈ U⎽(Λf).
+
+D[n-cov-inf-6]
+To prove that `a◃U` we have to exhibit the ordinal x such that `a ∈ U⎽x`.
+
+D[n-cov-inf-7]
+The definition of `U⎽(…+1)` expands to the union of two sets, and proving
+that `a ∈ X ∪ Y` is defined as proving that `a` is in `X` or `Y`. Applying
+the second constructor `@2;` of the disjunction, we are left to prove that `a` 
+belongs to the right hand side.
+
+D[n-cov-inf-8]
+We thus provide `i`, introduce the element being in the image and we are
+left to prove that it belongs to `U_(Λf)`. In the meanwhile, since belonging 
+to the image means that there exists an object in the domain… we eliminate the
+existential, obtaining `d` (of type `𝐃 a i`) and the equation defining `x`.  
+
+D[n-cov-inf-9]
+We just need to use the equational definition of `x` to obtain a conclusion
+that can be proved with `Hf'`. We assumed that `U_x` is extensional for
+every `x`, thus we are allowed to use `Hd` and close the proof.
+
+D*)
+
+(*D
+
+The next proof is that ◃(U) is mininal. The hardest part of the proof
+is to prepare the goal for the induction. The desiderata is to prove
+`U⎽o ⊆ V` by induction on `o`, but the conclusion of the lemma is,
+unfolding all definitions:
+
+> ∀x. x ∈ { y | ∃o:Ord A.y ∈ U⎽o } → x ∈ V
+
+D*)
+
+nlemma new_coverage_min :  
+  ∀A:nAx.∀U:Ω^A.∀V.U ⊆ V → (∀a:A.∀i.𝐈𝐦[𝐝 a i] ⊆ V → a ∈ V) → ◃U ⊆ V.
+#A; #U; #V; #HUV; #Im;#b;                       (** screenshot "n-cov-min-2". *)
+*; #o;                                          (** screenshot "n-cov-min-3". *)
+ngeneralize in match b; nchange with (U⎽o ⊆ V); (** screenshot "n-cov-min-4". *)
+nelim o;                                        (** screenshot "n-cov-min-5". *) 
+##[ #b; #bU0; napply HUV; napply bU0;
+##| #p; #IH; napply subseteq_union_l; ##[ nassumption; ##]
+    #x; *; #i; #H; napply (Im ? i); napply (subseteq_trans … IH); napply H;
+##| #a; #i; #f; #IH; #x; *; #d; napply IH; ##]
+nqed.
+
+(*D
+D[n-cov-min-2]
+After all the introductions, event the element hidden in the ⊆ definition,
+we have to eliminate the existential quantifier, obtaining the ordinal `o`
+
+D[n-cov-min-3]
+What is left is almost right, but the element `b` is already in the
+context. We thus generalize every occurrence of `b` in 
+the current goal, obtaining `∀c.c ∈ U⎽o → c ∈ V` that is `U⎽o ⊆ V`.
+
+D[n-cov-min-4]
+We then proceed by induction on `o` obtaining the following goals
+
+D[n-cov-min-5]
+All of them can be proved using simple set theoretic arguments,
+the induction hypothesis and the assumption `Im`.
+
+D*)
+
+
+(*D
+
+The notion `F_x` is again defined by recursion over the ordinal `x`.
+
+D*)
+
+nlet rec famF (A: nAx) (F : Ω^A) (x : Ord A) on x : Ω^A ≝ 
+  match x with
+  [ oO ⇒ F
+  | oS o ⇒ let F_o ≝ famF A F o in F_o ∩ { x | ∀i:𝐈 x.∃j:𝐃 x i.𝐝 x i j ∈ F_o } 
+  | oL a i f ⇒ { x | ∀j:𝐃 a i.x ∈ famF A F (f j) }
+  ].
+
+interpretation "famF" 'famU U x = (famF ? U x).
+
+ndefinition ord_fished : ∀A:nAx.∀F:Ω^A.Ω^A ≝ λA,F.{ y | ∀x:Ord A. y ∈ F⎽x }.
+
+interpretation "fished new fish" 'fished U = (ord_fished ? U).
+interpretation "new fish" 'fish a U = (mem ? (ord_fished ? U) a).
+
+(*D
+
+The proof of compatibility uses this little result, that we 
+proved outside the mail proof. 
+
+D*)
+nlemma co_ord_subset: ∀A:nAx.∀F:Ω^A.∀a,i.∀f:𝐃 a i → Ord A.∀j. F⎽(Λ f) ⊆ F⎽(f j).
+#A; #F; #a; #i; #f; #j; #x; #H; napply H;
+nqed.
+
+(*D
+
+We assume the dual of the axiom of choice, as in the paper proof.
+
+D*)
+naxiom AC_dual: ∀A:nAx.∀a:A.∀i,F. 
+ (∀f:𝐃 a i → Ord A.∃x:𝐃 a i.𝐝 a i x ∈ F⎽(f x)) → ∃j:𝐃 a i.∀x:Ord A.𝐝 a i j ∈ F⎽x.
+
+(*D
+
+Proving the anti-reflexivity property is simple, since the
+assumption `a ⋉ F` states that for every ordinal `x` (and thus also 0)
+`a ∈ F⎽x`. If `x` is choosen to be `0`, we obtain the thesis.
+
+D*)
+ntheorem new_fish_antirefl: ∀A:nAx.∀F:Ω^A.∀a. a ⋉ F → a ∈ F.
+#A; #F; #a; #H; nlapply (H 0); #aFo; napply aFo;
+nqed.
+
+(*D
+
+bar
+
+D*)
+ntheorem new_fish_compatible: 
+ ∀A:nAx.∀F:Ω^A.∀a. a ⋉ F → ∀i:𝐈 a.∃j:𝐃 a i.𝐝 a i j ⋉ F.
+#A; #F; #a; #aF; #i; nnormalize;               (** screenshot "n-f-compat-1". *)
+napply AC_dual; #f;                            (** screenshot "n-f-compat-2". *)
+nlapply (aF (Λf+1)); #aLf;                     (** screenshot "n-f-compat-3". *)
+nchange in aLf with 
+  (a ∈ F⎽(Λ f) ∧ ∀i:𝐈 a.∃j:𝐃 a i.𝐝 a i j ∈ F⎽(Λ f)); (** screenshot "n-f-compat-4". *)
+ncut (∃j:𝐃 a i.𝐝 a i j ∈ F⎽(f j));##[
+  ncases aLf; #_; #H; nlapply (H i);               (** screenshot "n-f-compat-5". *) 
+  *; #j; #Hj; @j; napply Hj;##] #aLf';             (** screenshot "n-f-compat-6". *)
+napply aLf';
+nqed.
+
+(*D
+D[n-f-compat-1]
+D[n-f-compat-2]
+D[n-f-compat-3]
+D[n-f-compat-4]
+D[n-f-compat-5]
+D[n-f-compat-6]
+
+D*)
+
+(*D
+
+The proof that `⋉(F)` is maximal is exactly the dual one of the
+minimality of `◃(U)`. Thus the main problem is to obtain `G ⊆ F⎽o`
+before doing the induction over `o`.
+
+D*)
+ntheorem max_new_fished: 
+  ∀A:nAx.∀G:ext_powerclass_setoid A.∀F:Ω^A.G ⊆ F → (∀a.a ∈ G → ∀i.𝐈𝐦[𝐝 a i] ≬ G) → G ⊆ ⋉F.
+#A; #G; #F; #GF; #H; #b; #HbG; #o; 
+ngeneralize in match HbG; ngeneralize in match b;
+nchange with (G ⊆ F⎽o);
+nelim o;
+##[ napply GF;
+##| #p; #IH; napply (subseteq_intersection_r … IH);
+    #x; #Hx; #i; ncases (H … Hx i); #c; *; *; #d; #Ed; #cG;
+    @d; napply IH;
+    alias symbol "prop2" = "prop21".
+napply (. Ed^-1‡#); napply cG;    
+##| #a; #i; #f; #Hf; nchange with (G ⊆ { y | ∀x. y ∈ F⎽(f x) }); 
+    #b; #Hb; #d; napply (Hf d); napply Hb;
+##]
+nqed.
+
+
+(*D
+<div id="appendix" class="anchor"></div>
+Appendix: tactics explanation
+-----------------------------
+
+In this appendix we try to give a description of tactics
+in terms of sequent calculus rules annotated with proofs.
+The `:` separator has to be read as _is a proof of_, in the spirit
+of the Curry-Howard isomorphism.
+
+                  Γ ⊢  f  :  A1 → … → An → B    Γ ⊢ ?1 : A1 … ?n  :  An 
+    napply f;    ⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼
+                           Γ ⊢ (f ?1 … ?n )  :  B
+ 
+                   Γ ⊢  ?  :  F → B       Γ ⊢ f  :  F 
+    nlapply f;    ⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼
+                             Γ ⊢ (? f)  :  B
+
+
+                 Γ; x : T  ⊢ ?  :  P(x) 
+    #x;      ⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼
+                Γ ⊢ λx:T.?  :  ∀x:T.P(x)
+
+                       
+                       Γ ⊢ ?_i  :  args_i → P(k_i args_i)          
+    ncases x;   ⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼
+                Γ ⊢ match x with [ k1 args1 ⇒ ?_1 | … | kn argsn ⇒ ?_n ]  :  P(x)                    
+
+
+                      Γ ⊢ ?i  :  ∀t. P(t) → P(k_i … t …)          
+    nelim x;   ⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼
+                   Γ ⊢ (T_rect_CProp0 ?_1 … ?_n)  :  P(x)                    
 
-theorem coreflexivity: ∀A:axiom_set.∀a:A.∀V. a ⋉ V → a ∈ V.
- intros;
- cases H;
- assumption.
-qed.
+Where `T_rect_CProp0` is the induction principle for the 
+inductive type `T`.
 
-theorem cotransitivity:
- ∀A:axiom_set.∀a:A.∀U,V. a ⋉ U → (∀b:A. b ⋉ U → b ∈ V) → a ⋉ V.
- intros;
- apply (fish_rec ?? {a|a ⋉ U} ??? H); simplify; intros;
-  [ apply H1; apply H2;
-  | cases H2 in j; clear H2; intro i;
-    cases (H4 i); clear H4; exists[apply a3] assumption]
-qed.
+                          Γ ⊢ ?  :  Q     Q ≡ P          
+    nchange with Q;   ⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼
+                          Γ ⊢ ?  :  P                    
 
-theorem compatibility: ∀A:axiom_set.∀a:A.∀U,V. a ⋉ V → a ◃ U → U ⋉ V.
- intros;
- generalize in match H; clear H; 
- apply (covers_elim ?? {a|a ⋉ V → U ⋉ V} ??? H1);
- clear H1; simplify; intros;
-  [ exists [apply x] assumption
-  | cases H2 in j H H1; clear H2 a1; intros; 
-    cases (H1 i); clear H1; apply (H3 a1); assumption]
-qed.
+Where the equivalence relation between types `≡` keeps into account
+β-reduction, δ-reduction (definition unfolding), ζ-reduction (local
+definition unfolding), ι-reduction (pattern matching simplification),
+μ-reduction (recursive function computation) and ν-reduction (co-fixpoint
+computation).
 
-definition leq ≝ λA:axiom_set.λa,b:A. a ◃ {y|b=y}.
 
-interpretation "covered by one" 'leq a b = (leq ? a b).
+                               Γ; H : Q; Δ ⊢ ?  :  G     Q ≡ P          
+    nchange in H with Q; ⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼
+                               Γ; H : P; Δ ⊢ ?  :  G                    
 
-theorem leq_refl: ∀A:axiom_set.∀a:A. a ≤ a.
- intros;
- apply refl;
- normalize;
- reflexivity.
-qed.
 
-theorem leq_trans: ∀A:axiom_set.∀a,b,c:A. a ≤ b → b ≤ c → a ≤ c.
- intros;
- unfold in H H1 ⊢ %;
- apply (transitivity ???? H);
- constructor 1;
- intros;
- normalize in H2;
- rewrite < H2;
- assumption.
-qed.
 
-definition uparrow ≝ λA:axiom_set.λa:A.mk_powerset ? (λb:A. a ≤ b).
+                    Γ ⊢ ?_2  :  T → G    Γ ⊢ ?_1  :  T
+    ncut T;   ⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼
+                               Γ ⊢ (?_2 ?_1)  :  G                    
 
-interpretation "uparrow" 'uparrow a = (uparrow ? a).
 
-definition downarrow ≝ λA:axiom_set.λU:Ω \sup A.mk_powerset ? (λa:A. (↑a) ≬ U).
+                                Γ ⊢ ?  :  ∀x.P(x)
+    ngeneralize in match t; ⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼⎼
+                                Γ ⊢ (? t)  :  P(t)
+                                
+D*)
 
-interpretation "downarrow" 'downarrow a = (downarrow ? a).
 
-definition fintersects ≝ λA:axiom_set.λU,V:Ω \sup A.↓U ∩ ↓V.
+(*D
 
-interpretation "fintersects" 'fintersects U V = (fintersects ? U V).
+[1]: http://upsilon.cc/~zack/research/publications/notation.pdf 
 
-record convergent_generated_topology : Type ≝
- { AA:> axiom_set;
-   convergence: ∀a:AA.∀U,V:Ω \sup AA. a ◃ U → a ◃ V → a ◃ (U ↓ V)
- }.
+D*)