reserve x for object;
reserve n for Nat;

theorem Th1:
  for X,Y,Z,T being set
  for x,y,z being object
  for f being Function of [:X,Y,Z:],T st x in X & y in Y & z in Z & T <> {}
  holds f.(x,y,z) in T
  proof
    let X,Y,Z,T be set;
    let x,y,z be object;
    let f be Function of [:X,Y,Z:],T;
    assume x in X & y in Y & z in Z;
    then [x,y,z] in [:X,Y,Z:] by MCART_1:69;
    hence thesis by FUNCT_2:5;
  end;
