External Filter Validator Function?

Well,

We may use the filter validators in models this way:

public function rules()
{
   return array
   (
      array('myAttribute', 'filter', 'filter' => array($this, 'someFunction')),
   }
}

public function someFunction($attribute)
{
   return 'Ho ho ho ' . $attribute;
}

This is working well presuming 'someFunction' is defined within the model class. But how to use a 'shared' library of filter functions - e.g. function stored in a helper file?

Any ideas? - Thanks ahead!