let I be set ; :: thesis: for X, Y being ManySortedSet of I holds X \+\ Y = (X \/ Y) \ (X /\ Y)
let X, Y be ManySortedSet of I; :: thesis: X \+\ Y = (X \/ Y) \ (X /\ Y)
thus X \+\ Y = (X \ (X /\ Y)) \/ (Y \ X) by Th76
.= (X \ (X /\ Y)) \/ (Y \ (X /\ Y)) by Th76
.= (X \/ Y) \ (X /\ Y) by Th78 ; :: thesis: verum