proposition: Lemma mem_catf f g k : k \in domf (f + g) = (k \in f) || (k \in g).
proof: Proof. by rewrite domf_cat inE. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)