2 include "arithmetics/bounded_quantifiers.ma".
3 include "basics/list.ma".
5 (* A bit of combinatorics *)
6 interpretation "list membership" 'mem a l = (mem ? a l).
8 lemma decidable_mem_nat: ∀n:nat.∀l. decidable (n ∈ l).
10 [%2 % @False_ind |#a #tl #Htl @decidable_or //]
13 lemma length_unique_le: ∀n,l. unique ? l → (∀x. x ∈ l → x < n) → |l| ≤ n.
15 [* // #a #tl #_ #H @False_ind @(absurd (a < 0))
16 [@H %1 % | @le_to_not_lt //]
17 |#m #Hind #l #Huni #Hmem <(filter_length2 ? (eqb m) l)
18 lapply (length_filter_eqb … m l Huni) #Hle
19 @(transitive_le ? (1+|filter ? (λx.¬ eqb m x) l|))
24 [@le_S_S_to_le @Hmem @(mem_filter … memx)] #Hcut
25 cases(le_to_or_lt_eq … Hcut) // #eqxm @False_ind
26 @(absurd ? eqxm) @sym_not_eq @eqb_false_to_not_eq
27 @injective_notb @(mem_filter_true ???? memx)
33 lemma eq_length_to_mem : ∀n,l. |l| = S n → unique ? l →
34 (∀x. x ∈ l → x ≤ n) → n ∈ l.
35 #n #l #H1 #H2 #H3 cases (decidable_mem_nat n l) //
36 #H4 @False_ind @(absurd (|l| > n))
38 |@le_to_not_lt @length_unique_le //
39 #x #memx cases(le_to_or_lt_eq … (H3 x memx)) //
40 #Heq @not_le_to_lt @(not_to_not … H4) #_ <Heq //
44 lemma eq_length_to_mem_all: ∀n,l. |l| = n → unique ? l →
45 (∀x. x ∈ l → x < n) → ∀i. i < n → i ∈ l.
47 [#l #_ #_ #_ #i #lti0 @False_ind @(absurd ? lti0 (not_le_Sn_O ?))
48 |#m #Hind #l #H #H1 #H2 #i #lei cases (le_to_or_lt_eq … lei)
49 [#leim @(mem_filter… (λi.¬(eqb m i)))
50 cases (filter_eqb m … H1)
51 [2: * #H @False_ind @(absurd ?? H) @eq_length_to_mem //
52 #x #memx @le_S_S_to_le @H2 //]
53 * #memm #Hfilter @Hind
54 [@injective_S <H <(filter_length2 ? (eqb m) l) >Hfilter %
56 |#x #memx cases (le_to_or_lt_eq … (H2 x (mem_filter … memx))) #H3
58 |@False_ind @(absurd (m=x)) [@injective_S //] @eqb_false_to_not_eq
59 @injective_notb >(mem_filter_true ???? memx) %
63 |#eqi @eq_length_to_mem >eqi [@H |@H1 |#x #Hx @le_S_S_to_le >eqi @H2 //]
68 lemma lt_length_to_not_mem: ∀n,l. unique ? l → (∀x. x ∈ l → x < n) → |l| < n →
69 ∃i. i < n ∧ ¬ (i ∈ l).
71 [#l #_ #_ #H @False_ind /2/
72 |#m #Hind #l #Huni #Hmem #Hlen cases (filter_eqb m … Huni)
74 |* #memm #Hfilter cases (Hind (filter ? (λx. ¬(eqb m x)) l) ? ? ?)
75 [#i * #ltim #memi %{i} % [@le_S // ]
76 @(not_to_not … memi) @mem_filter_l @injective_notb >notb_notb
77 @not_eq_to_eqb_false @sym_not_eq @lt_to_not_eq //
79 |#x #memx cases (le_to_or_lt_eq … (Hmem x ?))
81 |#H @False_ind @(absurd (m=x)) [@injective_S //] @eqb_false_to_not_eq
82 @injective_notb >(mem_filter_true ???? memx) %
85 |<(filter_length2 … (eqb m)) in Hlen; >Hfilter #H