consider a being set such that
A1: a in D1 by XBOOLE_0:def 1;
consider b being set such that
A2: b in D2 by XBOOLE_0:def 1;
reconsider a = a as Element of D1 by A1;
reconsider b = b as Element of D2 by A2;
a "\/" b in { (x "\/" y) where x, y is Element of L : ( x in D1 & y in D2 ) } ;
hence not D1 "\/" D2 is empty ; :: thesis: verum