Partial/PartialNode.php
private static function childNode(ArrayNode $node, $childNodeName)
    {
        if ($node instanceof PrototypedArrayNode && '*' === $childNodeName) {
            return self::nodePrototypeProperty()->getValue($node);
        }

        $children = self::nodeChildrenProperty()->getValue($node);

        if (!isset($children[$childNodeName])) {
            throw new UndefinedChildNode(
                $node,
                $childNodeName
            );
        }

        return $children[$childNodeName];
    }
private static function childNode(ArrayNode $node, $childNodeName)
    {
        if ($node instanceof PrototypedArrayNode && '*' === $childNodeName) {
            return self::nodePrototypeProperty()->getValue($node);
        }

        $children = self::nodeChildrenProperty()->getValue($node);

        if (!isset($children[$childNodeName])) {
            throw new UndefinedChildNode(
                $node,
                $childNodeName
            );
        }

        return $children[$childNodeName];
    }
@param ArrayNode $node
@param string    $childNodeName

@return NodeInterface