reserve P,Q,X,Y,Z for set, p,x,x9,x1,x2,y,z for object;
reserve D for non empty set;
reserve A,B for non empty set;
reserve Y for non empty set,
  f for Function of X,Y,
  p for PartFunc of Y,Z,
  x for Element of X;
reserve g for Function of X,X;
reserve X,Y for non empty set,
  Z,S,T for set,
  f for Function of X,Y,
  g for PartFunc of Y,Z,
  x for Element of X;

theorem Th121:
  Funcs(P,B) is FUNCTION_DOMAIN of P,B
proof
  for f be Element of Funcs(P,B) holds f is Function of P,B by Th65;
  hence thesis by Def12;
end;
