proposition: Lemma domf_cat f g : domf (f + g) = domf f `|` domf g.
proof: Proof. by apply/fsetP=> x; rewrite !inE; case: (boolP (_ \in domf _)); rewrite ?orbT. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)