proposition: Lemma unionE f1 f2 : union f1 f2 = to (UM.union (from f1) (from f2)).
proof: Proof. case: f1 f2=>[|f1 H1][|f2 H2] //; rewrite /union /UM.union /=. by case: ifP=>D //; rewrite heapE. Qed.
imports: Coq(ssreflect ssrbool ssrfun Eqdep) mathcomp(ssrnat eqtype seq path) pcm(options axioms finmap) pcm(pcm unionmap)