Section Eeq1.
Variables I X Y Z: Type.
Variable R' R1': relation2 X Y.
Variable S': relation2 Y Z.
Variable R  R1: relation2 X Y.
Variable S : relation2 Y Z.
Variables T' T: relation X.
Variables F' F: I -> relation2 X Y.

Lemma union2_incl: incl R R' -> incl R1 R1' -> incl (union2 R R1) (union2 R' R1').