theorem
for E being Element of sigma measurable_rectangles(S1,S2),
 f being E-measurable PartFunc of [:X1,X2:],ExtREAL
st M1 is sigma_finite & M2 is sigma_finite & E = dom f
holds f is_integrable_on Prod_Measure(M1,M2)
        iff Integral(M2,Integral1(M1,|.f.|)) < +infty by Lm1,Lm2;
