$where2 = $this->_cm_initialise($where, $selection, $search);
if ($where2 != $where) {
    // this was changed in _cm_initialise(), so use the new version
    $where = $where2;
} else {
    if (!empty($selection)) {
        // $selection takes precedence over $where
        $where     = $selection;
        $selection = null;
    } // if
} // if

    function _cm_initialise ($where, &$selection, $search)
    // perform any initialisation for the current task.
    // NOTE: $selection is passed by reference as it may be amended.
    // NOTE: $search    is only available for OUTPUT tasks.
    {
        // customisable code goes here

        return $where;

    } // _cm_initialise