definition replace_2 (A) (B): relation3 (relation2 A B) (relation A) (relation B) ≝
λR,Sa,Sb. ∀a1,b1. R a1 b1 → ∀a2. Sa a1 a2 → ∀b2. Sb b1 b2 → R a2 b2.
definition replace_2 (A) (B): relation3 (relation2 A B) (relation A) (relation B) ≝
λR,Sa,Sb. ∀a1,b1. R a1 b1 → ∀a2. Sa a1 a2 → ∀b2. Sb b1 b2 → R a2 b2.