proposition: Lemma fsub1 A a (aA : a \in A) : fsub A [fset a] = [set [` aA]] :> {set A}.
proof: Proof. by apply/setP=> x; rewrite !inE; congr eq_op. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)