proposition: Lemma in_fset1U a' A a : (a \in a' |` A) = (a == a') || (a \in A).
proof: Proof. by rewrite in_fsetU in_fset1. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)