reserve X for set;

theorem
  for A,B,C being Subset of X holds {A,B,C} is Subset-Family of X
proof
  let A,B,C be Subset of X;
  set D = {A,B,C};
  D c= bool X
  proof
    let x be object;
    assume x in D;
    then x = A or x = B or x = C by ENUMSET1:def 1;
    hence thesis;
  end;
  hence thesis;
end;
