for X1, X2 being non empty set
for S1 being SigmaField of X1
for S2 being SigmaField of X2
for F1 being Set_Sequence of S1
for F2 being Set_Sequence of S2
for n being Nat holds [:(F1 . n),(F2 . n):] is Element of sigma (measurable_rectangles (S1,S2))