code: public function addAlias($parameterName, $prefix = null)
      {
          if ($prefix == null) {
              $this->aliases[$this->prefix] = $parameterName;
          } else {
              $this->aliases[$prefix] = $parameterName;
          }
      }
docs: Add an alias and associate it with a prefix.
  If no prefix is defined, the default for
  the cluster will be used.
  
  Only one alias can exist per prefix per parameter.
  
  @param string $parameterName
  @param string $prefix