for X1, X2 being non empty set
for S1 being SigmaField of X1
for S2 being SigmaField of X2
for M1 being sigma_Measure of S1
for M2 being sigma_Measure of S2 holds sigma (measurable_rectangles (S1,S2)) c= sigma_Field (C_Meas (product_Measure (M1,M2)))