Section Union.
Variables X Y X' Y' I: Type.
Variable  RX: relation2 X X'.
Variables R: I -> relation2 X Y.
Variable  RY: relation2 Y Y'.
Variable  S: relation2 X' Y'.
Hypothesis H: forall i, diagram RX (R i) RY S.

Theorem diagram_union: diagram RX (union R) RY S.