theorem
  [:X1,X2,X3:] = the set of all  [x1,x2,x3]
proof
  defpred P[set,set,set] means not contradiction;
A1: for x being Element of [:X1,X2,X3:] holds x in the set of all  [x1,x2,x3]
  proof
    let x be Element of [:X1,X2,X3:];
    x = [x`1_3,x`2_3,x`3_3];
    hence thesis;
  end;
  for X1,X2,X3 holds { [x1,x2,x3] : P[x1,x2,x3] } is Subset of [:X1,X2,X3
  :] from Fraenkel3;
  then the set of all  [x1,x2,x3]  is Subset of [:X1,X2,X3:];
  hence thesis by A1,SUBSET_1:28;
end;
