
theorem Th11:
  TWOELEMENTSETS {} = {}
proof
  TWOELEMENTSETS {} c= {}
  proof
    let a be object;
    assume a in TWOELEMENTSETS({});
    then
    ex a1,a2 being object st a1 in {} & a2 in {} &( not a1=a2)& a = {a1,a2}
by Th8;
    hence thesis;
  end;
  hence thesis;
end;
