reserve X for set;

theorem
  for A,B being Subset of X holds {A,B} is Subset-Family of X
proof
  let A,B be Subset of X;
  set C = {A,B};
  C c= bool X
  proof
    let x be object;
    assume x in C;
    then x = A or x = B by TARSKI:def 2;
    hence thesis;
  end;
  hence thesis;
end;
