Section Incl.
Variables X X' Y Y': Type.
Variable RX: relation2 X X'.
Variable R R':  relation2 X Y.
Variable RY: relation2 Y Y'.
Variables S S': relation2 X' Y'.
Hypothesis H: diagram RX R RY S.
Hypothesis HR: forall x y, R' x y -> R x y.
Hypothesis HS: forall x y, S x y -> S' x y.

Theorem diagram_incl: diagram RX R' RY S'.