proposition: Lemma symmetric A B (x:A) (Ra: relation A) (Rb: relation B) (y:B) `{!Symmetric Ra}: R Ra x _ Rb y → R Rb y _ Ra x.
proof: Proof. destruct 1. apply relate. symmetry. assumption. Qed.
imports: Relation_Definitions Setoid Unicode.Utf8 Utf8_core