matita/matita/contribs/lambdadelta/delayed_updating/syntax/path_head.ma

   1 (**************************************************************************)
   2 (*       ___                                                              *)
   3 (*      ||M||                                                             *)
   4 (*      ||A||       A project by Andrea Asperti                           *)
   5 (*      ||T||                                                             *)
   6 (*      ||I||       Developers:                                           *)
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   8 (*      ||A||         http://helm.cs.unibo.it                             *)
   9 (*      \   /                                                             *)
  10 (*       \ /        This file is distributed under the terms of the       *)
  11 (*        v         GNU General Public License Version 2                  *)
  12 (*                                                                        *)
  13 (**************************************************************************)
  14
  15 include "delayed_updating/syntax/path_labels.ma".
  16 include "delayed_updating/notation/functions/downarrowright_2.ma".
  17 include "ground/arith/nat_plus.ma".
  18 include "ground/arith/nat_pred_succ.ma".
  19
  20 (* HEAD FOR PATH ************************************************************)
  21
  22 rec definition path_head (m) (p) on p: path ≝
  23 match m with
  24 [ nzero  ⇒ 𝐞
  25 | ninj o ⇒
  26   match p with
  27   [ list_empty     ⇒ 𝗟∗∗m
  28   | list_lcons l q ⇒
  29     match l with
  30     [ label_d n ⇒ l◗(path_head (m+n) q)
  31     | label_m   ⇒ l◗(path_head m q)
  32     | label_L   ⇒ l◗(path_head (↓o) q)
  33     | label_A   ⇒ l◗(path_head m q)
  34     | label_S   ⇒ l◗(path_head m q)
  35     ]
  36   ]
  37 ].
  38
  39 interpretation
  40   "head (reversed path)"
  41   'DownArrowRight n p = (path_head n p).
  42
  43 (* basic constructions ****************************************************)
  44
  45 lemma path_head_zero (p):
  46       (𝐞) = ↳[𝟎]p.
  47 * // qed.
  48
  49 lemma path_head_empty (n):
  50       (𝗟∗∗n) = ↳[n]𝐞.
  51 * // qed.
  52
  53 lemma path_head_d_sn (p) (n) (m:pnat):
  54       (𝗱m◗↳[↑n+m]p) = ↳[↑n](𝗱m◗p).
  55 // qed.
  56
  57 lemma path_head_m_sn (p) (n):
  58       (𝗺◗↳[↑n]p) = ↳[↑n](𝗺◗p).
  59 // qed.
  60
  61 lemma path_head_L_sn (p) (n):
  62       (𝗟◗↳[n]p) = ↳[↑n](𝗟◗p).
  63 #p #n
  64 whd in ⊢ (???%); //
  65 qed.
  66
  67 lemma path_head_A_sn (p) (n):
  68       (𝗔◗↳[↑n]p) = ↳[↑n](𝗔◗p).
  69 // qed.
  70
  71 lemma path_head_S_sn (p) (n):
  72       (𝗦◗↳[↑n]p) = ↳[↑n](𝗦◗p).
  73 // qed.
  74
  75 (* Basic inversions *********************************************************)
  76
  77 lemma eq_inv_path_empty_head (p) (n):
  78       (𝐞) = ↳[n]p → 𝟎 = n.
  79 *
  80 [ #m <path_head_empty #H0
  81   <(eq_inv_empty_labels … H0) -m //
  82 | * [ #n ] #p #n @(nat_ind_succ … n) -n // #m #_
  83   [ <path_head_d_sn
  84   | <path_head_m_sn
  85   | <path_head_L_sn
  86   | <path_head_A_sn
  87   | <path_head_S_sn
  88   ] #H0 destruct
  89 ]
  90 qed-.