theorem Th33: :: YELLOW_3:33

for S1, S2 being non empty RelStr
for D1 being Subset of S1
for D2 being Subset of S2
for x being Element of S1
for y being Element of S2 st x is_<=_than D1 & y is_<=_than D2 holds
[x,y] is_<=_than [:D1,D2:]