proposition: Lemma remf_cat f g A : (f + g).[\ A] = f.[\ A] + g.[\ A].
proof: Proof. by apply/fmapP => k'; rewrite !(fnd_cat, fnd_rem) mem_remf; case: (_ \in _). Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)