protected function libJoin($args)
{
list($list1, $list2, $sep) = $args;
$list1 = $this->coerceList($list1, ' ');
$list2 = $this->coerceList($list2, ' ');
$sep = $this->listSeparatorForJoin($list1, $sep);
return [Type::T_LIST, $sep, array_merge($list1[2], $list2[2])];
}