* array(baseKeyVal => array(subKeyVal => [ignored], ...), ...)
* @param string $baseKey Field name to match the base-level keys to (eg 'pl_namespace')
* @param string $subKey Field name to match the sub-level keys to (eg 'pl_title')
* array(baseKeyVal => array(subKeyVal => [ignored], ...), ...)
* @param string $baseKey Field name to match the base-level keys to (eg 'pl_namespace')
* @param string $subKey Field name to match the sub-level keys to (eg 'pl_title')