
theorem Th1:
  for X being set, Y being non empty set, f being Function of X,Y,
  A being Subset of X st f is one-to-one holds f".:(f.:A)=A
proof
  let X be set, Y be non empty set, f be Function of X,Y, A be Subset of X;
A1: dom f = X by FUNCT_2:def 1;
  assume f is one-to-one;
  hence thesis by A1,FUNCT_1:107;
end;
