<?php declare(strict_types = 1);

/**
 * @template TParent of ParentNodeInterface
 */
interface ChildNodeInterface
{
    /** @return TParent */
    public function getParent(): ParentNodeInterface;
}

interface ParentNodeInterface
{
    /** @return list<ChildNodeInterface<static>> */
    public function getChildren(): array;
}

final class Block implements ParentNodeInterface
{
    /** @var list<Row> */
    private $rows = [];
	
    /** @return list<Row> */
    public function getChildren(): array
    {
        return $this->rows;
    }	
}

/** @implements ChildNodeInterface<Block> */
final class Row implements ChildNodeInterface
{
    /** @var Block $parent */
    private $parent;
	
    public function getParent(): Block {
        return $this->parent;
    }
}