public override string to_string () {
string s = "";
switch (level) {
case ONE:
case Group.Level.ONE:
s = "(";
break;
case TWO:
case Group.Level.TWO :
s = "[";
case THREE:
case Group.Level.THREE:
s = "{";
}
s += e.to_string ();
s += ")";
s += "]";
s += "}";