reserve X,Y,Z,Z1,Z2,D for set,x,y for object;

theorem Th2:
  meet X c= union X
proof
A1: now
    assume
A2: X <> {};
    now
      set y = the Element of X;
      let x be object;
      assume x in meet X;
      then x in y by A2,Def1;
      hence x in union X by A2,TARSKI:def 4;
    end;
    hence thesis;
  end;
  X = {} implies thesis by Def1;
  hence thesis by A1;
end;
