:: deftheorem defines Measurable-Y-section MEASUR11:def 7 :
for X1, X2 being non empty set
for S1 being SigmaField of X1
for S2 being SigmaField of X2
for E being Element of sigma (measurable_rectangles (S1,S2))
for y being set holds Measurable-Y-section (E,y) = Y-section (E,y);