proposition: Lemma in_fsetD1 A b a : (a \in A `\ b) = (a != b) && (a \in A).
proof: Proof. by rewrite in_fsetD in_fset1. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)