/**
  * Get the full version of a namespace prefix
  *
  * Looks up a prefix (atom:, etc.) in the list of registered
  * namespaces and returns the full namespace URI if
  * available. Returns the prefix, unmodified, if it's not
  * registered.
  *
  * The current entry or feed's version will be used when performing the
  * namespace lookup unless overridden using $majorVersion and
  * $minorVersion. If the entry/fee has a null version, then the latest
  * protocol version will be used by default.
  *
  * @param string $prefix The namespace prefix to lookup.
  * @param integer $majorVersion The major protocol version in effect.
  *        Defaults to null (auto-select).
  * @param integer $minorVersion The minor protocol version in effect.
  *        Defaults to null (auto-select).
  * @return string
  */
 public function lookupNamespace($prefix, $majorVersion = null, $minorVersion = null)
 {
     // Auto-select current version
     if ($majorVersion === null) {
         $majorVersion = $this->getMajorProtocolVersion();
     }
     if ($minorVersion === null) {
         $minorVersion = $this->getMinorProtocolVersion();
     }
     // Perform lookup
     return parent::lookupNamespace($prefix, $majorVersion, $minorVersion);
 }