theorem Th44:
  for D1,D2,D being non empty set, f being Function of [:D1,D2:],D for
X1 being Subset of D1, X2 being Subset of D2 holds (f.:^2).(X1,X2) = f.:[:X1,X2
  :]
proof
  let D1,D2,D be non empty set, f be Function of [:D1,D2:],D;
  let X1 be Subset of D1, X2 be Subset of D2;
  [X1,X2]`1 = X1 & [X1,X2]`2 = X2;
  hence thesis by Def8;
end;
