proposition: Lemma can2_imset_pre : cancel f g -> cancel g f -> f @: D = g @^-1: D.
proof: Proof. by move=> fK gK; apply: can2_in_imset_pre; apply: in1W. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop)