proposition: Lemma set_join1 x : set_join [set x] = x.
proof: Proof. by apply/le_anti; rewrite set_join_ub // andbT set_join_le_ub // ub_set1. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype choice order) mathcomp(ssrnat bigop classical_sets)