private static function isMissingVariable($valueNode, $variables)
    {
        return $valueNode instanceof VariableNode &&
            (count($variables) === 0 || ! array_key_exists($valueNode->name->value, $variables));
    }
private static function isMissingVariable($valueNode, $variables)
    {
        return $valueNode instanceof VariableNode &&
            (count($variables) === 0 || ! array_key_exists($valueNode->name->value, $variables));
    }
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