theorem
  for X1, X2 being non empty SubSpace of X, f1 being Function of X1,Y,
f2 being Function of X2,Y st X1 misses X2 or f1|(X1 meet X2) = f2|(X1 meet X2)
  holds f1 union f2 = f2 union f1
proof
  let X1, X2 be non empty SubSpace of X, f1 be Function of X1,Y, f2 be
  Function of X2,Y;
  assume X1 misses X2 or f1|(X1 meet X2) = f2|(X1 meet X2);
  then (f1 union f2)|X1 = f1 & (f1 union f2)|X2 = f2 by Def12;
  hence thesis by Th126;
end;
