proposition: Lemma class_is_im_of_singleton x: class r x = im_of_singleton r x.
proof: Proof. set_extens t; first by move/class_P/dirim_set1_P. by move/dirim_set1_P/class_P. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)