let X, Y be set ; :: thesis: proj1_3 (X \/ Y) = (proj1_3 X) \/ (proj1_3 Y)
thus proj1_3 (X \/ Y) = proj1 ((proj1 X) \/ (proj1 Y)) by Th23
.= (proj1_3 X) \/ (proj1_3 Y) by Th23 ; :: thesis: verum