[TYPO3-english] wfqbe: using processCheckInsertValues hook

[Claudio Strizzolo]

Hi all,
I'm trying to use the processCheckInsertValues hook provided by wfqbe 
extension. Unfortunately I could not find any example of that hook, then 
I'm trying to figure it out myself. Basically I just need it to validate 
some dates and little more.
In particular, I do not know what the custom_validation_insert_values is 
expected to return, and how I can display an explaination of the reason 
why a validation failed, in case. I tried by looking at the code, but 
without success.
Up to now, the following is my code. It does not work, of course:

(...)
class tx_wfqbe_processCheckInsertValues {

function tx_wfqbe_processCheckInsertValues() {
}

function custom_validation_insert_values(&$wfqbe, $piVars, &$callingObj) {
 
  $out='';

  $date=$wfqbe[0];

  (validate the $date here)
  if (something goes wrong) {
    $out.='The date you provided is not valid';                    
  }

  return $out;
}

}


Any hints?
Thanks in advance,

Claudio