Fix compilation on Coq 8.3.

lib/coq/list/Length.v

@@ -19,7 +19,7 @@ Proof.
 intros a a_WT l.
 induction l.
 easy.
-change (1 + length l = Z.of_nat (S (List.length l)))%Z.
+change (1 + length l = Z_of_nat (S (List.length l)))%Z.
 now rewrite inj_S, Zplus_comm, IHl.
 Qed.
 
@@ -39,6 +39,7 @@ Proof.
 intros a a_WT [|h t] ; split ; try easy.
 unfold length. fold length.
 intros H.
+exfalso.
 generalize (Length_nonnegative t).
 omega.
 Qed.

lib/coq/list/NthHdTl.v

@@ -19,6 +19,8 @@ easy.
 simpl.
 generalize (Zeq_bool_if (i + 1) 0).
 case Zeq_bool.
+intro H.
+exfalso.
 omega.
 intros _.
 simpl in h1.

lib/coq/list/NthLength.v

@@ -41,6 +41,7 @@ generalize (Zeq_bool_if i 0).
 case Zeq_bool.
 intros H'.
 rewrite H' in H.
+exfalso.
 generalize (Length.Length_nonnegative q).
 omega.
 intros _.

lib/coq/list/NthLengthAppend.v

@@ -49,6 +49,7 @@ simpl.
 generalize (Zeq_bool_if i 0).
 case Zeq_bool.
 intros Hi'.
+exfalso.
 generalize (Length.Length_nonnegative l1).
 omega.
 intros _.