code: private static function isMissingVariable($valueNode, $variables)
      {
          return $valueNode instanceof VariableNode &&
              (count($variables) === 0 || ! array_key_exists($valueNode->name->value, $variables));
      }
docs: Returns true if the provided valueNode is a variable which is not defined
  in the set of variables.
  
  @param ValueNode $valueNode
  @param mixed[]   $variables
  
  @return bool