+
+definition bi_symmetric: ∀A,B. ∀R: bi_relation A B. Prop ≝ λA,B,R.
+ ∀a1,a2,b1,b2. R a2 b2 a1 b1 → R a1 b1 a2 b2.
+
+definition bi_transitive: ∀A,B. ∀R: bi_relation A B. Prop ≝ λA,B,R.
+ ∀a1,a,b1,b. R a1 b1 a b →
+ ∀a2,b2. R a b a2 b2 → R a1 b1 a2 b2.