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4 (* ||A|| A project by Andrea Asperti *)
6 (* ||I|| Developers: *)
7 (* ||T|| The HELM team. *)
8 (* ||A|| http://helm.cs.unibo.it *)
10 (* \ / This file is distributed under the terms of the *)
11 (* v GNU General Public License Version 2 *)
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15 include "ground_2/notation/constructors/cons_2.ma".
16 include "ground_2/notation/relations/exteq_3.ma".
17 include "ground_2/lib/star.ma".
19 (* STREAMS ******************************************************************)
21 coinductive stream (A:Type[0]): Type[0] ≝
22 | seq: A → stream A → stream A
25 interpretation "cons (nstream)" 'Cons b t = (seq ? b t).
27 coinductive eq_stream (A): relation (stream A) ≝
28 | eq_seq: ∀t1,t2,b1,b2. b1 = b2 → eq_stream A t1 t2 → eq_stream A (b1@t1) (b2@t2)
31 interpretation "extensional equivalence (nstream)"
32 'ExtEq A t1 t2 = (eq_stream A t1 t2).
34 definition eq_stream_repl_back (A) (R:predicate …) ≝
35 ∀t1,t2. t1 ≐⦋A⦌ t2 → R t1 → R t2.
37 definition eq_stream_repl_fwd (A) (R:predicate …) ≝
38 ∀t1,t2. t2 ≐⦋A⦌ t1 → R t1 → R t2.
40 (* Basic inversion lemmas ***************************************************)
42 fact eq_stream_inv_seq_aux: ∀A,t1,t2. t1 ≐⦋A⦌ t2 →
43 ∀u1,u2,a1,a2. t1 = a1@u1 → t2 = a2@u2 →
46 #t1 #t2 #b1 #b2 #Hb #Ht #u1 #u2 #a1 #a2 #H1 #H2 destruct /2 width=1 by conj/
49 lemma eq_stream_inv_seq: ∀A,t1,t2,b1,b2. b1@t1 ≐⦋A⦌ b2@t2 → b1 = b2 ∧ t1 ≐ t2.
50 /2 width=5 by eq_stream_inv_seq_aux/ qed-.
52 (* Basic properties *********************************************************)
54 lemma stream_expand (A) (t:stream A): t = match t with [ seq a u ⇒ a @ u ].
58 let corec eq_stream_refl: ∀A. reflexive … (eq_stream A) ≝ ?.
62 let corec eq_stream_sym: ∀A. symmetric … (eq_stream A) ≝ ?.
64 #t1 #t2 #b1 #b2 #Hb #Ht @eq_seq /2 width=1 by/
67 lemma eq_stream_repl_sym: ∀A,R. eq_stream_repl_back A R → eq_stream_repl_fwd A R.
68 /3 width=3 by eq_stream_sym/ qed-.
70 (* Main properties **********************************************************)
72 let corec eq_stream_trans: ∀A. Transitive … (eq_stream A) ≝ ?.
74 #t1 #t #b1 #b #Hb1 #Ht1 * #b2 #t2 #H cases (eq_stream_inv_seq A … H) -H
75 #Hb2 #Ht2 @eq_seq /2 width=3 by/
78 theorem eq_stream_canc_sn: ∀A,t,t1,t2. t ≐ t1 → t ≐ t2 → t1 ≐⦋A⦌ t2.
79 /3 width=3 by eq_stream_trans, eq_stream_sym/ qed-.
81 theorem eq_stream_canc_dx: ∀A,t,t1,t2. t1 ≐ t → t2 ≐ t → t1 ≐⦋A⦌ t2.
82 /3 width=3 by eq_stream_trans, eq_stream_sym/ qed-.