proposition: Lemma dom_mapE x : dom (dom_map x) = dom x.
proof: Proof. by apply: dom_omap_some. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path bigop) pcm(options prelude finmap seqperm pred seqext) pcm(pcm morphism)