(* Merge *)
Definition merge (uf: t) (a b: elt) : t :=
let a' := repr uf a in
let b' := repr uf b in
match M.elt_eq a' b' with
| left EQ => uf
| right NEQ => identify uf a' b (repr_res_none uf a) (not_eq_sym NEQ)
end.

Lemma sameclass_merge:
forall uf a b x y, sameclass (merge uf a b) x y <-> sameclass (union uf a b) x y.