proposition: Lemma If_form_qf : qf_form pred_f -> qf_form then_f -> qf_form else_f -> qf_form If.
proof: Proof. by move=> /= -> -> ->. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)