
theorem Th1:
for X1,X2 be non empty set, S1 be SigmaField of X1, S2 be SigmaField of X2
 holds
  sigma DisUnion measurable_rectangles(S1,S2)
    = sigma measurable_rectangles(S1,S2)
proof
   let X1,X2 be non empty set, S1 be SigmaField of X1, S2 be SigmaField of X2;
   Field_generated_by (measurable_rectangles(S1,S2))
    = DisUnion (measurable_rectangles(S1,S2)) by SRINGS_3:22;
   hence thesis by SRINGS_3:23;
end;
