/**
  * Returns true if and only if $value meets the validation requirements
  *
  * If $value fails validation, then this method returns false, and
  * getMessages() will return an array of messages that explain why the
  * validation failed.
  *
  * @param  mixed $value
  * @param  mixed $content
  * @return boolean
  * @throws \Zend_Validate_Exception If validation of $value is impossible
  */
 public function isValid($value, $context = array())
 {
     $result = $this->_user->authenticate($value);
     return $this->setAuthResult($result);
 }