public boolean add(symbol sym) throws internal_error {
  Object previous;
  not_null(sym);
  previous=_all.put(sym.name(),sym);
  return previous == null;
}

public boolean add ( symbol sym ) throws internal_error { Object previous ; not_null ( sym ) ; previous = _all . put ( sym . name ( ) , sym ) ; return previous == null ; }
add a single symbol to the set .
