(** * Merging two trees *)

Lemma merge_avl_1 : forall m1 m2, avl m1 -> avl m2 ->
-(2) <= height m1 - height m2 <= 2 ->
avl (merge m1 m2) /\
0<= height (merge m1 m2) - max (height m1) (height m2) <=1.