proposition: Lemma fset_cons a s : [fset[key] x in a :: s] = a |` [fset[key] x in s].
proof: Proof. by apply/fsetP=> x; rewrite in_fset_cons !inE. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)