theorem Th09: :: MEASUR10:11

for X1, X2 being set
for S1 being semialgebra_of_sets of X1
for S2 being semialgebra_of_sets of X2 holds { [:A,B:] where A is Element of S1, B is Element of S2 : verum } is semialgebra_of_sets of [:X1,X2:]