proposition: Lemma remf1_id f k : k \notin f -> f.[~ k] = f.
proof: Proof. by move=> kNf; rewrite remf_id //= fdisjointX1. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)