
theorem
  for X being set
  st (for Y being object st Y in X holds Y is Graph-membered set)
  holds union X is Graph-membered
proof
  let X be set;
  assume A1: for Y being object st Y in X holds Y is Graph-membered set;
  let x be object;
  assume x in union X;
  then consider Y being set such that
    A2: x in Y & Y in X by TARSKI:def 4;
  thus thesis by A1, A2, Def1;
end;
