let X, Y be set ; :: thesis: proj2_4 (X \/ Y) = (proj2_4 X) \/ (proj2_4 Y)
thus proj2_4 (X \/ Y) = proj2 ((proj1_3 X) \/ (proj1_3 Y)) by Th31
.= (proj2_4 X) \/ (proj2_4 Y) by Th27 ; :: thesis: verum