1 | include "basics/types.ma". |
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2 | include "arithmetics/nat.ma". |
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3 | |
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4 | inductive nat_compared : nat → nat → Type[0] ≝ |
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5 | | nat_lt : ∀n,m:nat. nat_compared n (n+S m) |
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6 | | nat_eq : ∀n:nat. nat_compared n n |
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7 | | nat_gt : ∀n,m:nat. nat_compared (m+S n) m. |
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8 | |
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9 | let rec nat_compare (n:nat) (m:nat) : nat_compared n m ≝ |
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10 | match n return λx. nat_compared x m with |
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11 | [ O ⇒ match m return λy. nat_compared O y with [ O ⇒ nat_eq ? | S m' ⇒ nat_lt ?? ] |
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12 | | S n' ⇒ |
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13 | match m return λy. nat_compared (S n') y with |
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14 | [ O ⇒ nat_gt n' O |
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15 | | S m' ⇒ match nat_compare n' m' return λx,y.λ_. nat_compared (S x) (S y) with |
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16 | [ nat_lt x y ⇒ nat_lt ?? |
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17 | | nat_eq x ⇒ nat_eq ? |
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18 | | nat_gt x y ⇒ nat_gt ? (S y) |
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19 | ] |
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20 | ] |
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21 | ]. |
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22 | |
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23 | lemma nat_compare_eq : ∀n. nat_compare n n = nat_eq n. |
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24 | #n elim n |
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25 | [ @refl |
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26 | | #m #IH whd in ⊢ (??%?); >IH @refl |
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27 | ] qed. |
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28 | |
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29 | lemma nat_compare_lt : ∀n,m. nat_compare n (n+S m) = nat_lt n m. |
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30 | #n #m elim n |
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31 | [ // |
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32 | | #n' #IH whd in ⊢ (??%?); >IH @refl |
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33 | ] qed. |
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34 | |
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35 | lemma nat_compare_gt : ∀n,m. nat_compare (m+S n) m = nat_gt n m. |
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36 | #n #m elim m |
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37 | [ // |
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38 | | #m' #IH whd in ⊢ (??%?); >IH @refl |
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39 | ] qed. |
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40 | |
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41 | |
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42 | let rec eq_nat_dec (n:nat) (m:nat) : Sum (n=m) (n≠m) ≝ |
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43 | match n return λn.Sum (n=m) (n≠m) with |
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44 | [ O ⇒ match m return λm.Sum (O=m) (O≠m) with [O ⇒ inl ?? (refl ??) | S m' ⇒ inr ??? ] |
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45 | | S n' ⇒ match m return λm.Sum (S n'=m) (S n'≠m) with [O ⇒ inr ??? | S m' ⇒ |
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46 | match eq_nat_dec n' m' with [ inl E ⇒ inl ??? | inr NE ⇒ inr ??? ] ] |
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47 | ]. |
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48 | [ 1,2: % #E destruct |
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49 | | >E @refl |
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50 | | % #E destruct cases NE /2/ |
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51 | ] qed. |
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52 | |
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53 | lemma max_l : ∀m,n,o:nat. o ≤ m → o ≤ max m n. |
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54 | #m #n #o #H whd in ⊢ (??%); @leb_elim #H' |
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55 | [ @(transitive_le ? m ? H H') |
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56 | | @H |
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57 | ] qed. |
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58 | |
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59 | lemma max_r : ∀m,n,o:nat. o ≤ n → o ≤ max m n. |
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60 | #m #n #o #H whd in ⊢ (??%); @leb_elim #H' |
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61 | [ @H |
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62 | | @(transitive_le … H) @(transitive_le … (not_le_to_lt … H')) // |
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63 | ] qed. |
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64 | |
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65 | lemma le_S_to_le: ∀n,m:ℕ.S n ≤ m → n ≤ m. |
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66 | /2/ qed. |
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67 | |
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68 | lemma le_plus_k: |
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69 | ∀n,m:ℕ.n ≤ m → ∃k:ℕ.m = n + k. |
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70 | #n #m elim m -m; |
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71 | [ #Hn % [ @O | <(le_n_O_to_eq n Hn) // ] |
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72 | | #m #Hind #Hn cases (le_to_or_lt_eq … Hn) -Hn; #Hn |
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73 | [ elim (Hind (le_S_S_to_le … Hn)) #k #Hk % [ @(S k) | >Hk // ] |
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74 | | % [ @O | <Hn // ] |
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75 | ] |
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76 | ] |
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77 | qed. |
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78 | |
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79 | lemma eq_plus_S_to_lt: |
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80 | ∀n,m,p:ℕ.n = m + (S p) → m < n. |
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81 | #n #m #p /2 by lt_plus_to_lt_l/ |
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82 | qed. |
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83 | |
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84 | (* "Fast" proofs: some proofs get reduced during normalization (in particular, |
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85 | some functions which use a proof for rewriting are applied to constants and |
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86 | get reduced during a proof or while matita is searching for a term; |
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87 | they may also be normalized during testing), and so here are some more |
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88 | efficient versions. Perhaps they could be replaced using some kind of proof |
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89 | irrelevance? *) |
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90 | |
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91 | let rec plus_n_Sm_fast (n:nat) on n : ∀m:nat. S (n+m) = n+S m ≝ |
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92 | match n return λn'.∀m.S(n'+m) = n'+S m with |
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93 | [ O ⇒ λm.refl ?? |
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94 | | S n' ⇒ λm. ? |
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95 | ]. normalize @(match plus_n_Sm_fast n' m with [ refl ⇒ ? ]) @refl qed. |
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96 | |
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97 | let rec plus_n_O_faster (n:nat) : n = n + O ≝ |
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98 | match n return λn.n=n+O with |
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99 | [ O ⇒ refl ?? |
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100 | | S n' ⇒ match plus_n_O_faster n' return λx.λ_.S n'=S x with [ refl ⇒ refl ?? ] |
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101 | ]. |
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102 | |
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103 | let rec commutative_plus_faster (n,m:nat) : n+m = m+n ≝ |
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104 | match n return λn.n+m = m+n with |
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105 | [ O ⇒ plus_n_O_faster ? |
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106 | | S n' ⇒ ? |
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107 | ]. @(match plus_n_Sm_fast m n' return λx.λ_. ? = x with [ refl ⇒ ? ]) |
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108 | @(match commutative_plus_faster n' m return λx.λ_.? = S x with [refl ⇒ ?]) @refl qed. |
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109 | |
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110 | lemma distributive_times_plus_fast : distributive ? times plus. |
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111 | #n elim n [ #m #p % ] |
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112 | #n' #IH #m #p normalize |
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113 | >IH |
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114 | >associative_plus in ⊢ (???%); |
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115 | <(associative_plus ? p) in ⊢ (???%); |
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116 | >(commutative_plus_faster ? p) in ⊢ (???%); |
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117 | >(associative_plus p) |
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118 | @associative_plus |
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119 | qed. |
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120 | |
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121 | lemma times_n_Sm_fast : ∀n,m.n + n * m = n * S m. |
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122 | #n elim n -n |
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123 | [ #m % ] |
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124 | #n #IH #m normalize <IH |
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125 | <associative_plus >(commutative_plus_faster n) |
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126 | >associative_plus >IH % |
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127 | qed. |
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128 | |
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129 | lemma commutative_times_fast : commutative ? times. |
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130 | #n elim n -n |
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131 | [ #m <times_n_O % ] |
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132 | #n #IH #m normalize <times_n_Sm_fast >IH % |
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133 | qed. |
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134 | |
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