let I be set ; :: thesis: for X, Y being ManySortedSet of I holds X \/ (X \/ Y) = X \/ Y
let X, Y be ManySortedSet of I; :: thesis: X \/ (X \/ Y) = X \/ Y
thus X \/ (X \/ Y) = (X \/ X) \/ Y by Th28
.= X \/ Y ; :: thesis: verum