proposition: Lemma fsfun_dflt f k : k \notin finsupp f -> f k = default k.
proof: Proof. by rewrite mem_finsupp negbK => /eqP. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)