public function idArithmetic($action, array $options = array())
    {
        return $this->_idArithmetic($this->id, $action, array_merge(array(
            'count' => 1
        ), $options));
    }
public function idArithmetic($action, array $options = array())
    {
        return $this->_idArithmetic($this->id, $action, array_merge(array(
            'count' => 1
        ), $options));
    }
public function idArithmetic ( $ action , array $ options = array ( ) ) { return $ this -> _idArithmetic ( $ this -> id , $ action , array_merge ( array ( 'count' => 1 ) , $ options ) ) ; }
Performs MIME ID "arithmetic".

@param string $action  One of:
- ID_DOWN: ID of child. Note: ID_DOWN will first traverse to "$id.0"
if given an ID *NOT* of the form "$id.0". If given an ID of
the form "$id.0", ID_DOWN will traverse to "$id.1". This
behavior can be avoided if 'no_rfc822' option is set.
- ID_NEXT: ID of next sibling.
- ID_PREV: ID of previous sibling.
- ID_UP: ID of parent. Note: ID_UP will first traverse to "$id.0" if
given an ID *NOT* of the form "$id.0". If given an ID of the
form "$id.0", ID_UP will traverse to "$id". This behavior can
be avoided if 'no_rfc822' option is set.
@param array $options  Additional options:
- count: (integer) How many levels to traverse.
DEFAULT: 1
- no_rfc822: (boolean) Don't traverse RFC 822 sub-levels.
DEFAULT: false

@return mixed  The resulting ID string, or null if that ID can not
exist.
Performs MIME ID arithmetic .