function parse_production()
{
$head = $this->parse_terminal('token');
$this->parse_terminal('colon');
$body = $this->parse_body();
$p = new production($head);
$p->add($body);
while ($this->can_parse_terminal('union')) {
$this->parse_terminal('union');
$p->add($this->parse_body());
}
return $p;
}
