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diff --git a/weblib/arithmetics/pidgeon_hole.ma b/weblib/arithmetics/pidgeon_hole.ma
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+
+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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