--- /dev/null
+
+include "arithmetics/bounded_quantifiers.ma".
+include "basics/list.ma".
+
+(* A bit of combinatorics *)
+interpretation "list membership" 'mem a l = (mem ? a l).
+
+lemma decidable_mem_nat: ∀n:nat.∀l. decidable (n ∈ l).
+#n #l elim l
+ [%2 % @False_ind |#a #tl #Htl @decidable_or //]
+qed.
+
+lemma length_unique_le: ∀n,l. unique ? l → (∀x. x ∈ l → x < n) → |l| ≤ n.
+#n elim n
+ [* // #a #tl #_ #H @False_ind @(absurd (a < 0))
+ [@H %1 % | @le_to_not_lt //]
+ |#m #Hind #l #Huni #Hmem <(filter_length2 ? (eqb m) l)
+ lapply (length_filter_eqb … m l Huni) #Hle
+ @(transitive_le ? (1+|filter ? (λx.¬ eqb m x) l|))
+ [@le_plus //
+ |@le_S_S @Hind
+ [@unique_filter //
+ |#x #memx cut (x ≤ m)
+ [@le_S_S_to_le @Hmem @(mem_filter … memx)] #Hcut
+ cases(le_to_or_lt_eq … Hcut) // #eqxm @False_ind
+ @(absurd ? eqxm) @sym_not_eq @eqb_false_to_not_eq
+ @injective_notb @(mem_filter_true ???? memx)
+ ]
+ ]
+ ]
+qed.
+
+lemma eq_length_to_mem : ∀n,l. |l| = S n → unique ? l →
+ (∀x. x ∈ l → x ≤ n) → n ∈ l.
+#n #l #H1 #H2 #H3 cases (decidable_mem_nat n l) //
+#H4 @False_ind @(absurd (|l| > n))
+ [>H1 //
+ |@le_to_not_lt @length_unique_le //
+ #x #memx cases(le_to_or_lt_eq … (H3 x memx)) //
+ #Heq @not_le_to_lt @(not_to_not … H4) #_ <Heq //
+ ]
+qed.
+
+lemma eq_length_to_mem_all: ∀n,l. |l| = n → unique ? l →
+ (∀x. x ∈ l → x < n) → ∀i. i < n → i ∈ l.
+#n elim n
+ [#l #_ #_ #_ #i #lti0 @False_ind @(absurd ? lti0 (not_le_Sn_O ?))
+ |#m #Hind #l #H #H1 #H2 #i #lei cases (le_to_or_lt_eq … lei)
+ [#leim @(mem_filter… (λi.¬(eqb m i)))
+ cases (filter_eqb m … H1)
+ [2: * #H @False_ind @(absurd ?? H) @eq_length_to_mem //
+ #x #memx @le_S_S_to_le @H2 //]
+ * #memm #Hfilter @Hind
+ [@injective_S <H <(filter_length2 ? (eqb m) l) >Hfilter %
+ |@unique_filter @H1
+ |#x #memx cases (le_to_or_lt_eq … (H2 x (mem_filter … memx))) #H3
+ [@le_S_S_to_le @H3
+ |@False_ind @(absurd (m=x)) [@injective_S //] @eqb_false_to_not_eq
+ @injective_notb >(mem_filter_true ???? memx) %
+ ]
+ |@le_S_S_to_le @leim
+ ]
+ |#eqi @eq_length_to_mem >eqi [@H |@H1 |#x #Hx @le_S_S_to_le >eqi @H2 //]
+ ]
+ ]
+qed.
+
+lemma lt_length_to_not_mem: ∀n,l. unique ? l → (∀x. x ∈ l → x < n) → |l| < n →
+∃i. i < n ∧ ¬ (i ∈ l).
+#n elim n
+ [#l #_ #_ #H @False_ind /2/
+ |#m #Hind #l #Huni #Hmem #Hlen cases (filter_eqb m … Huni)
+ [2: * #H #_ %{m} % //
+ |* #memm #Hfilter cases (Hind (filter ? (λx. ¬(eqb m x)) l) ? ? ?)
+ [#i * #ltim #memi %{i} % [@le_S // ]
+ @(not_to_not … memi) @mem_filter_l @injective_notb >notb_notb
+ @not_eq_to_eqb_false @sym_not_eq @lt_to_not_eq //
+ |@unique_filter //
+ |#x #memx cases (le_to_or_lt_eq … (Hmem x ?))
+ [#H @le_S_S_to_le @H
+ |#H @False_ind @(absurd (m=x)) [@injective_S //] @eqb_false_to_not_eq
+ @injective_notb >(mem_filter_true ???? memx) %
+ |@(mem_filter … memx)
+ ]
+ |<(filter_length2 … (eqb m)) in Hlen; >Hfilter #H
+ @le_S_S_to_le @H
+ ]
+ ]
+ ]
+qed.
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