public boolean add(terminal sym) throws internal_error {
  boolean result;
  not_null(sym);
  result=_elements.get(sym.index());
  if (!result)   _elements.set(sym.index());
  return result;
}

public boolean add ( terminal sym ) throws internal_error { boolean result ; not_null ( sym ) ; result = _elements . get ( sym . index ( ) ) ; if ( ! result ) _elements . set ( sym . index ( ) ) ; return result ; }
add a single terminal to the set .
