private function _parseConstructorArgument(__ConfigurationSection $property, __InstanceDefinition &$instance_definition)
 {
     if ($property->hasAttribute('index')) {
         $argument_index = $property->getAttribute('index');
     } else {
         $argument_index = null;
     }
     $argument_value = $this->_parsePropertyValue($property, 'constructor argument', $instance_definition->getId());
     $instance_definition->addConstructorArgument($argument_value, $argument_index);
 }