
theorem Th1:
  for X,Y being set for f being Function of [:X,Y:],X holds dom f = [:X,Y:]
proof
  let X,Y be set;
  let f be Function of [:X,Y:],X;
  X is empty implies [:X,Y:] is empty by ZFMISC_1:90;
  hence thesis by FUNCT_2:def 1;
end;
