X-Git-Url: http://matita.cs.unibo.it/gitweb/?a=blobdiff_plain;f=helm%2Fsoftware%2Fmatita%2Fcontribs%2Fdama%2Fdama%2Fsupremum.ma;h=0de61b292fcaca3e40f5b7bddfa7ba0b88d27398;hb=7a9277a3775b7150a22b2039548508e85751f85a;hp=65d43ac18d3f62be9278426b008e4a2b08a5e2db;hpb=20c68603cc71aeb957d5e974558d908f2261fb6d;p=helm.git

diff --git a/helm/software/matita/contribs/dama/dama/supremum.ma b/helm/software/matita/contribs/dama/dama/supremum.ma
index 65d43ac18..0de61b292 100644
--- a/helm/software/matita/contribs/dama/dama/supremum.ma
+++ b/helm/software/matita/contribs/dama/dama/supremum.ma
@@ -12,92 +12,144 @@
 (*                                                                        *)
 (**************************************************************************)
 
+
+include "datatypes/constructors.ma".
+include "nat/plus.ma".
+include "nat_ordered_set.ma".
 include "sequence.ma".
-include "ordered_set.ma".
 
 (* Definition 2.4 *)
-definition upper_bound ≝ λO:ordered_set.λa:sequence O.λu:O.∀n:nat.a n ≤ u.
+definition upper_bound ≝ 
+  λO:half_ordered_set.λa:sequence O.λu:O.∀n:nat.a n ≤≤ u.
 
 definition supremum ≝
-  λO:ordered_set.λs:sequence O.λx.upper_bound ? s x ∧ (∀y:O.x ≰ y → ∃n.s n ≰ y).
+  λO:half_ordered_set.λs:sequence O.λx.
+    upper_bound ? s x ∧ (∀y:O.x ≰≰ y → ∃n.s n ≰≰ y).
 
-definition increasing ≝ λO:ordered_set.λa:sequence O.∀n:nat.a n ≤ a (S n).
+definition increasing ≝ 
+  λO:half_ordered_set.λa:sequence O.∀n:nat.a n ≤≤ a (S n).
 
-notation < "x \nbsp 'is_upper_bound' \nbsp s" non associative with precedence 50 
+notation < "x \nbsp 'is_upper_bound' \nbsp s" non associative with precedence 45 
   for @{'upper_bound $s $x}.
-notation < "s \nbsp 'is_increasing'"          non associative with precedence 50 
+notation < "x \nbsp 'is_lower_bound' \nbsp s" non associative with precedence 45 
+  for @{'lower_bound $s $x}.
+notation < "s \nbsp 'is_increasing'" non associative with precedence 45 
   for @{'increasing $s}.
-notation < "x \nbsp 'is_supremum' \nbsp s"  non associative with precedence 50 
+notation < "s \nbsp 'is_decreasing'" non associative with precedence 45 
+  for @{'decreasing $s}.
+notation < "x \nbsp 'is_supremum' \nbsp s" non associative with precedence 45 
   for @{'supremum $s $x}.
-
-notation > "x 'is_upper_bound' s" non associative with precedence 50 
+notation < "x \nbsp 'is_infimum' \nbsp s" non associative with precedence 45 
+  for @{'infimum $s $x}.
+notation > "x 'is_upper_bound' s" non associative with precedence 45 
   for @{'upper_bound $s $x}.
-notation > "s 'is_increasing'"    non associative with precedence 50 
+notation > "x 'is_lower_bound' s" non associative with precedence 45 
+  for @{'lower_bound $s $x}.
+notation > "s 'is_increasing'"    non associative with precedence 45 
   for @{'increasing $s}.
-notation > "x 'is_supremum' s"  non associative with precedence 50 
+notation > "s 'is_decreasing'"    non associative with precedence 45 
+  for @{'decreasing $s}.
+notation > "x 'is_supremum' s"  non associative with precedence 45 
   for @{'supremum $s $x}.
+notation > "x 'is_infimum' s"  non associative with precedence 45 
+  for @{'infimum $s $x}.
 
-interpretation "Ordered set upper bound" 'upper_bound s x = 
-  (cic:/matita/dama/supremum/upper_bound.con _ s x).
-interpretation "Ordered set increasing"  'increasing s    = 
-  (cic:/matita/dama/supremum/increasing.con _ s).
-interpretation "Ordered set strong sup"  'supremum s x  = 
-  (cic:/matita/dama/supremum/supremum.con _ s x).
-  
-include "bishop_set.ma".
-  
-lemma uniq_supremum: 
-  ∀O:ordered_set.∀s:sequence O.∀t1,t2:O.
-    t1 is_supremum s → t2 is_supremum s → t1 ≈ t2.
-intros (O s t1 t2 Ht1 Ht2); cases Ht1 (U1 H1); cases Ht2 (U2 H2); 
-apply le_le_eq; intro X;
-[1: cases (H1 ? X); apply (U2 w); assumption
-|2: cases (H2 ? X); apply (U1 w); assumption]
-qed.
+interpretation "Ordered set upper bound" 'upper_bound s x = (upper_bound (os_l _) s x).
+interpretation "Ordered set lower bound" 'lower_bound s x = (upper_bound (os_r _) s x).
+
+interpretation "Ordered set increasing"  'increasing s = (increasing (os_l _) s).
+interpretation "Ordered set decreasing"  'decreasing s = (increasing (os_r _) s).
 
+interpretation "Ordered set strong sup"  'supremum s x = (supremum (os_l _) s x).
+interpretation "Ordered set strong inf"  'infimum s x = (supremum (os_r _) s x).
+  
 (* Fact 2.5 *)
-lemma supremum_is_upper_bound: 
-  ∀C:ordered_set.∀a:sequence C.∀u:C.
-   u is_supremum a → ∀v.v is_upper_bound a → u ≤ v.
+lemma h_supremum_is_upper_bound: 
+  ∀C:half_ordered_set.∀a:sequence C.∀u:C.
+   supremum ? a u → ∀v.upper_bound ? a v → u ≤≤ v.
 intros 7 (C s u Hu v Hv H); cases Hu (_ H1); clear Hu;
-cases (H1 ? H) (w Hw); apply Hv; assumption;
+cases (H1 ? H) (w Hw); apply Hv; [apply w] assumption;
 qed.
 
+notation "'supremum_is_upper_bound'" non associative with precedence 90 for @{'supremum_is_upper_bound}.
+notation "'infimum_is_lower_bound'" non associative with precedence 90 for @{'infimum_is_lower_bound}.
+
+interpretation "supremum_is_upper_bound" 'supremum_is_upper_bound = (h_supremum_is_upper_bound (os_l _)).
+interpretation "infimum_is_lower_bound" 'infimum_is_lower_bound = (h_supremum_is_upper_bound (os_r _)).
+
 (* Lemma 2.6 *)
 definition strictly_increasing ≝ 
-  λC:ordered_set.λa:sequence C.∀n:nat.a (S n) ≰ a n.
- 
-notation < "s \nbsp 'is_strictly_increasing'" non associative with precedence 50 
+  λC:half_ordered_set.λa:sequence C.∀n:nat.a (S n) ≰≰ a n.
+
+notation < "s \nbsp 'is_strictly_increasing'" non associative with precedence 45 
   for @{'strictly_increasing $s}.
-notation > "s 'is_strictly_increasing'" non associative with precedence 50 
+notation > "s 'is_strictly_increasing'" non associative with precedence 45 
   for @{'strictly_increasing $s}.
-interpretation "Ordered set increasing"  'strictly_increasing s    = 
-  (cic:/matita/dama/supremum/strictly_increasing.con _ s).
+interpretation "Ordered set strict increasing"  'strictly_increasing s    = 
+  (strictly_increasing (os_l _) s).
   
-notation "a \uparrow u" non associative with precedence 50 for @{'sup_inc $a $u}.
-interpretation "Ordered set supremum of increasing" 'sup_inc s u =
- (cic:/matita/dama/cprop_connectives/And.ind#xpointer(1/1) 
-  (cic:/matita/dama/supremum/increasing.con _ s)
-  (cic:/matita/dama/supremum/supremum.con _ s u)).
+notation < "s \nbsp 'is_strictly_decreasing'" non associative with precedence 45 
+  for @{'strictly_decreasing $s}.
+notation > "s 'is_strictly_decreasing'" non associative with precedence 45 
+  for @{'strictly_decreasing $s}.
+interpretation "Ordered set strict decreasing"  'strictly_decreasing s    = 
+  (strictly_increasing (os_r _) s).
 
-include "nat_ordered_set.ma".
-  
-alias symbol "nleq" = "Ordered set excess".
-alias symbol "leq" = "Ordered set less or equal than".
-lemma trans_increasing: 
-  ∀C:ordered_set.∀a:sequence C.a is_increasing → ∀n,m:nat_ordered_set. m ≰ n → a n ≤ a m.
+definition uparrow ≝
+  λC:half_ordered_set.λs:sequence C.λu:C.
+   increasing ? s ∧ supremum ? s u.
+
+interpretation "Ordered set uparrow" 'funion s u = (uparrow (os_l _) s u).
+interpretation "Ordered set downarrow" 'fintersects s u = (uparrow (os_r _) s u).
+
+lemma h_trans_increasing: 
+  ∀C:half_ordered_set.∀a:sequence C.increasing ? a → 
+   ∀n,m:nat_ordered_set. n ≤ m → a n ≤≤ a m.
+intros 5 (C a Hs n m); elim m; [
+  rewrite > (le_n_O_to_eq n (not_lt_to_le O n H));
+  intro X; cases (hos_coreflexive ? (a n) X);]
+cases (le_to_or_lt_eq ?? (not_lt_to_le (S n1) n H1)); clear H1;
+[2: rewrite > H2; intro; cases (hos_coreflexive ? (a (S n1)) H1);
+|1: apply (hle_transitive ???? (H ?) (Hs ?));
+    intro; whd in H1; apply (not_le_Sn_n n); apply (transitive_le ??? H2 H1);]
+qed.
+
+notation "'trans_increasing'" non associative with precedence 90 for @{'trans_increasing}.
+notation "'trans_decreasing'" non associative with precedence 90 for @{'trans_decreasing}.
+
+interpretation "trans_increasing" 'trans_increasing = (h_trans_increasing (os_l _)).
+interpretation "trans_decreasing" 'trans_decreasing = (h_trans_increasing (os_r _)).
+
+lemma hint_nat :
+ Type_of_ordered_set nat_ordered_set →
+   hos_carr (os_l (nat_ordered_set)).
+intros; assumption;
+qed.
+
+coercion hint_nat nocomposites.
+
+lemma h_trans_increasing_exc: 
+  ∀C:half_ordered_set.∀a:sequence C.increasing ? a → 
+   ∀n,m:nat_ordered_set. m ≰≰ n → a n ≤≤ a m.
 intros 5 (C a Hs n m); elim m; [cases (not_le_Sn_O n H);]
 intro; apply H; 
-[1: change in n1 with (os_carr nat_ordered_set); (* canonical structures *) 
-    change with (n<n1); (* is sort elimination not allowed preserved by delta? *)
+[1: change in n1 with (hos_carr (os_l nat_ordered_set)); 
+    change with (n<n1);
     cases (le_to_or_lt_eq ?? H1); [apply le_S_S_to_le;assumption]
-    cases (Hs n); rewrite < H3 in H2; assumption (* ogni goal di tipo Prop non è anche di tipo CProp *)    
-|2: cases (os_cotransitive ??? (a n1) H2); [assumption]
+    cases (Hs n); rewrite < H3 in H2; assumption;    
+|2: cases (hos_cotransitive ? (a n) (a (S n1)) (a n1) H2); [assumption]
     cases (Hs n1); assumption;]
 qed.
 
-lemma strictly_increasing_reaches: 
-  ∀C:ordered_set.∀m:sequence nat_ordered_set.
+notation "'trans_increasing_exc'" non associative with precedence 90 for @{'trans_increasing_exc}.
+notation "'trans_decreasing_exc'" non associative with precedence 90 for @{'trans_decreasing_exc}.
+
+interpretation "trans_increasing_exc" 'trans_increasing_exc = (h_trans_increasing_exc (os_l _)).
+interpretation "trans_decreasing_exc" 'trans_decreasing_exc = (h_trans_increasing_exc (os_r _)).
+
+alias symbol "exists" = "CProp exists".
+lemma nat_strictly_increasing_reaches: 
+  ∀m:sequence nat_ordered_set.
    m is_strictly_increasing → ∀w.∃t.m t ≰ w.
 intros; elim w;
 [1: cases (nat_discriminable O (m O)); [2: cases (not_le_Sn_n O (ltn_to_ltO ?? H1))]
@@ -112,15 +164,279 @@ intros; elim w;
         apply (transitive_le ??? H2 Hp);]]
 qed.
      
-lemma selection: 
-  ∀C:ordered_set.∀m:sequence nat_ordered_set.m is_strictly_increasing →
-    ∀a:sequence C.∀u.a ↑ u → (λx.a (m x)) ↑ u.
+lemma h_selection_uparrow: 
+  ∀C:half_ordered_set.∀m:sequence nat_ordered_set.
+   m is_strictly_increasing →
+    ∀a:sequence C.∀u.uparrow ? a u → uparrow ? ⌊x,a (m x)⌋ u.
 intros (C m Hm a u Ha); cases Ha (Ia Su); cases Su (Uu Hu); repeat split; 
-[1: intro n; simplify; apply trans_increasing; [assumption] apply (Hm n);
+[1: intro n; simplify; apply (h_trans_increasing_exc ? a Ia); apply (Hm n);
 |2: intro n; simplify; apply Uu;
 |3: intros (y Hy); simplify; cases (Hu ? Hy);
-    cases (strictly_increasing_reaches C ? Hm w); 
-    exists [apply w1]; cases (os_cotransitive ??? (a (m w1)) H); [2:assumption]  
-    cases (trans_increasing C ? Ia ?? H1); assumption;]
+    cases (nat_strictly_increasing_reaches ? Hm w); 
+    exists [apply w1]; cases (hos_cotransitive ? (a w) y (a (m w1)) H); [2:assumption]  
+    cases (h_trans_increasing_exc ?? Ia w (m w1) H1); assumption;]
 qed.     
+
+notation "'selection_uparrow'" non associative with precedence 90 for @{'selection_uparrow}.
+notation "'selection_downarrow'" non associative with precedence 90 for @{'selection_downarrow}.
+
+interpretation "selection_uparrow" 'selection_uparrow = (h_selection_uparrow (os_l _)).
+interpretation "selection_downarrow" 'selection_downarrow = (h_selection_uparrow (os_r _)).
+
+(* Definition 2.7 *)
+definition order_converge ≝
+  λO:ordered_set.λa:sequence O.λx:O.
+   exT23 (sequence O) (λl.l ↑ x) (λu.u ↓ x)
+     (λl,u:sequence O.∀i:nat. (l i) is_infimum ⌊w,a (w+i)⌋ ∧ 
+                   (u i) is_supremum ⌊w,a (w+i)⌋).
     
+notation < "a \nbsp (\cir \atop (\horbar\triangleright)) \nbsp x" non associative with precedence 45 
+  for @{'order_converge $a $x}.
+notation > "a 'order_converges' x" non associative with precedence 45 
+  for @{'order_converge $a $x}.
+interpretation "Order convergence" 'order_converge s u = (order_converge _ s u).   
+    
+(* Definition 2.8 *)
+record segment (O : Type) : Type ≝ {
+   seg_l_ : O;
+   seg_u_ : O
+}.
+
+notation > "𝕦_term 90 s" non associative with precedence 90 for @{'upp $s}.
+notation "𝕦 \sub term 90 s" non associative with precedence 90 for @{'upp $s}. 
+notation > "𝕝_term 90 s" non associative with precedence 90 for @{'low $s}.
+notation "𝕝 \sub term 90 s" non associative with precedence 90 for @{'low $s}. 
+ 
+definition seg_u ≝
+ λO:half_ordered_set.λs:segment O.
+   wloss O ?? (λl,u.l) (seg_u_ ? s) (seg_l_ ? s).
+definition seg_l ≝
+ λO:half_ordered_set.λs:segment O.
+   wloss O ?? (λl,u.l) (seg_l_ ? s) (seg_u_ ? s). 
+ 
+interpretation "uppper" 'upp s = (seg_u (os_l _) s).
+interpretation "lower" 'low s = (seg_l (os_l _) s).
+interpretation "uppper dual" 'upp s = (seg_l (os_r _) s).
+interpretation "lower dual" 'low s = (seg_u (os_r _) s).
+ 
+definition in_segment ≝ 
+  λO:half_ordered_set.λs:segment O.λx:O.
+    wloss O ?? (λp1,p2.p1 ∧ p2) (seg_l ? s ≤≤ x) (x ≤≤ seg_u ? s).
+
+notation "‡O" non associative with precedence 90 for @{'segment $O}.
+interpretation "Ordered set sergment" 'segment x = (segment x).
+
+interpretation "Ordered set sergment in" 'mem x s = (in_segment _ s x).
+
+definition segment_ordered_set_carr ≝
+  λO:half_ordered_set.λs:‡O.∃x.x ∈ s.
+definition segment_ordered_set_exc ≝ 
+  λO:half_ordered_set.λs:‡O.
+   λx,y:segment_ordered_set_carr O s.hos_excess_ O (\fst x) (\fst y).
+lemma segment_ordered_set_corefl:
+ ∀O,s. coreflexive ? (wloss O ?? (segment_ordered_set_exc O s)).
+intros 3; cases x; cases (wloss_prop O);
+generalize in match (hos_coreflexive O w);
+rewrite < (H1 ?? (segment_ordered_set_exc O s));
+rewrite < (H1 ?? (hos_excess_ O)); intros; assumption;
+qed.
+lemma segment_ordered_set_cotrans : 
+  ∀O,s. cotransitive ? (wloss O ?? (segment_ordered_set_exc O s)).
+intros 5 (O s x y z); cases x; cases y ; cases z; clear x y z;
+generalize in match (hos_cotransitive O w w1 w2);
+cases (wloss_prop O); 
+do 3 rewrite < (H3 ?? (segment_ordered_set_exc O s));
+do 3 rewrite < (H3 ?? (hos_excess_ O)); intros; apply H4; assumption;
+qed.  
+  
+lemma half_segment_ordered_set: 
+  ∀O:half_ordered_set.∀s:segment O.half_ordered_set.
+intros (O a); constructor 1;
+[ apply (segment_ordered_set_carr O a);
+| apply (wloss O);
+| apply (wloss_prop O);
+| apply (segment_ordered_set_exc O a);
+| apply (segment_ordered_set_corefl O a);
+| apply (segment_ordered_set_cotrans ??);
+]
+qed.
+
+lemma segment_ordered_set: 
+  ∀O:ordered_set.∀s:‡O.ordered_set.
+intros (O s); 
+apply half2full; apply (half_segment_ordered_set (os_l O) s); 
+qed.
+
+notation "{[ term 19 s ]}" non associative with precedence 90 for @{'segset $s}.
+interpretation "Ordered set segment" 'segset s = (segment_ordered_set _ s). 
+
+(* test :
+ ∀O:ordered_set.∀s: segment (os_l O).∀x:O.
+   in_segment (os_l O) s x
+   =
+   in_segment (os_r O) s x.
+intros; try reflexivity;
+*)
+
+lemma prove_in_segment: 
+ ∀O:half_ordered_set.∀s:segment O.∀x:O.
+   (seg_l O s) ≤≤ x → x ≤≤ (seg_u O s) → x ∈ s.
+intros; unfold; cases (wloss_prop O); rewrite < H2;
+split; assumption;
+qed.
+
+lemma cases_in_segment: 
+  ∀C:half_ordered_set.∀s:segment C.∀x. x ∈ s → (seg_l C s) ≤≤ x ∧ x ≤≤ (seg_u C s).
+intros; unfold in H; cases (wloss_prop C) (W W); rewrite<W in H; [cases H; split;assumption]
+cases H; split; assumption;
+qed. 
+
+definition hint_sequence: 
+  ∀C:ordered_set.
+    sequence (hos_carr (os_l C)) → sequence (Type_of_ordered_set C).
+intros;assumption;
+qed.
+
+definition hint_sequence1: 
+  ∀C:ordered_set.
+    sequence (hos_carr (os_r C)) → sequence (Type_of_ordered_set_dual C).
+intros;assumption;
+qed.
+
+definition hint_sequence2: 
+  ∀C:ordered_set.
+    sequence (Type_of_ordered_set C) → sequence (hos_carr (os_l C)).
+intros;assumption;
+qed.
+
+definition hint_sequence3: 
+  ∀C:ordered_set.
+    sequence (Type_of_ordered_set_dual C) → sequence (hos_carr (os_r C)).
+intros;assumption;
+qed.
+
+coercion hint_sequence nocomposites.
+coercion hint_sequence1 nocomposites.
+coercion hint_sequence2 nocomposites.
+coercion hint_sequence3 nocomposites.
+
+(* Lemma 2.9 - non easily dualizable *)
+
+lemma x2sx_:
+  ∀O:half_ordered_set.
+   ∀s:segment O.∀x,y:half_segment_ordered_set ? s.
+    \fst x ≰≰ \fst y → x ≰≰ y.
+intros 4; cases x; cases y; clear x y; simplify; unfold hos_excess;
+whd in ⊢ (?→? (% ? ?)? ? ? ? ?); simplify in ⊢ (?→%);
+cases (wloss_prop O) (E E); do 2 rewrite < E; intros; assumption;
+qed.
+
+lemma sx2x_:
+  ∀O:half_ordered_set.
+   ∀s:segment O.∀x,y:half_segment_ordered_set ? s.
+    x ≰≰ y → \fst x ≰≰ \fst y.
+intros 4; cases x; cases y; clear x y; simplify; unfold hos_excess;
+whd in ⊢ (? (% ? ?) ?? ? ? ? → ?); simplify in ⊢ (% → ?);
+cases (wloss_prop O) (E E); do 2 rewrite < E; intros; assumption;
+qed.
+
+lemma l2sl_:
+  ∀C,s.∀x,y:half_segment_ordered_set C s. \fst x ≤≤ \fst y → x ≤≤ y.
+intros; intro; apply H; apply sx2x_; apply H1;
+qed.
+
+
+lemma sl2l_:
+  ∀C,s.∀x,y:half_segment_ordered_set C s. x ≤≤ y → \fst x ≤≤ \fst y.
+intros; intro; apply H; apply x2sx_; apply H1;
+qed.
+
+coercion x2sx_ nocomposites.
+coercion sx2x_ nocomposites.
+coercion l2sl_ nocomposites.
+coercion sl2l_ nocomposites.
+
+lemma h_segment_preserves_supremum:
+  ∀O:half_ordered_set.∀s:segment O.
+   ∀a:sequence (half_segment_ordered_set ? s).
+    ∀x:half_segment_ordered_set ? s. 
+      increasing ? ⌊n,\fst (a n)⌋ ∧ 
+      supremum ? ⌊n,\fst (a n)⌋ (\fst x) → uparrow ? a x.
+intros; split; cases H; clear H; 
+[1: intro n; lapply (H1 n) as K; clear H1 H2;
+    intro; apply K; clear K; apply rule H; 
+|2: cases H2; split; clear H2;
+    [1: intro n; lapply (H n) as K; intro W; apply K;
+        apply rule W;
+    |2: clear H1 H; intros (y0 Hy0); cases (H3 (\fst y0));[exists[apply w]]
+        [1: change in H with (\fst (a w) ≰≰ \fst y0); apply rule H;
+        |2: apply rule Hy0;]]]
+qed.
+
+notation "'segment_preserves_supremum'" non associative with precedence 90 for @{'segment_preserves_supremum}.
+notation "'segment_preserves_infimum'" non associative with precedence 90 for @{'segment_preserves_infimum}.
+
+interpretation "segment_preserves_supremum" 'segment_preserves_supremum = (h_segment_preserves_supremum (os_l _)).
+interpretation "segment_preserves_infimum" 'segment_preserves_infimum = (h_segment_preserves_supremum (os_r _)).
+
+(*
+test segment_preserves_infimum2:
+  ∀O:ordered_set.∀s:‡O.∀a:sequence {[s]}.∀x:{[s]}. 
+    ⌊n,\fst (a n)⌋ is_decreasing ∧ 
+    (\fst x) is_infimum ⌊n,\fst (a n)⌋ → a ↓ x.
+intros; apply (segment_preserves_infimum s a x H);
+qed.
+*)
+       
+(* Definition 2.10 *)
+
+alias symbol "pi2" = "pair pi2".
+alias symbol "pi1" = "pair pi1".
+(*
+definition square_segment ≝ 
+  λO:half_ordered_set.λs:segment O.λx: square_half_ordered_set O.
+    in_segment ? s (\fst x) ∧ in_segment ? s (\snd x).
+*) 
+definition convex ≝
+  λO:half_ordered_set.λU:square_half_ordered_set O → Prop.
+    ∀s.U s → le O (\fst s) (\snd s) → 
+     ∀y. 
+       le O (\fst y) (\snd s) → 
+       le O (\fst s) (\fst y) →
+       le O (\snd y) (\snd s) →
+       le O (\fst y) (\snd y) →
+       U y.
+  
+(* Definition 2.11 *)  
+definition upper_located ≝
+  λO:half_ordered_set.λa:sequence O.∀x,y:O. y ≰≰ x → 
+    (∃i:nat.a i ≰≰ x) ∨ (∃b:O.y ≰≰ b ∧ ∀i:nat.a i ≤≤ b).
+
+notation < "s \nbsp 'is_upper_located'" non associative with precedence 45 
+  for @{'upper_located $s}.
+notation > "s 'is_upper_located'" non associative with precedence 45 
+  for @{'upper_located $s}.
+interpretation "Ordered set upper locatedness" 'upper_located s = 
+  (upper_located (os_l _) s).
+
+notation < "s \nbsp 'is_lower_located'" non associative with precedence 45 
+  for @{'lower_located $s}.
+notation > "s 'is_lower_located'" non associative with precedence 45
+  for @{'lower_located $s}.
+interpretation "Ordered set lower locatedness" 'lower_located s = 
+  (upper_located (os_r _) s).
+      
+(* Lemma 2.12 *)    
+lemma h_uparrow_upperlocated:
+  ∀C:half_ordered_set.∀a:sequence C.∀u:C.uparrow ? a u → upper_located ? a.
+intros (C a u H); cases H (H2 H3); clear H; intros 3 (x y Hxy);
+cases H3 (H4 H5); clear H3; cases (hos_cotransitive C y x u Hxy) (W W);
+[2: cases (H5 x W) (w Hw); left; exists [apply w] assumption;
+|1: right; exists [apply u]; split; [apply W|apply H4]]
+qed.
+
+notation "'uparrow_upperlocated'" non associative with precedence 90 for @{'uparrow_upperlocated}.
+notation "'downarrow_lowerlocated'" non associative with precedence 90 for @{'downarrow_lowerlocated}.
+
+interpretation "uparrow_upperlocated" 'uparrow_upperlocated = (h_uparrow_upperlocated (os_l _)).
+interpretation "downarrow_lowerlocated" 'downarrow_lowerlocated = (h_uparrow_upperlocated (os_r _)).