peekBlockStack example

switch ($name) {
            case 'parent':
                $this->parseArguments();
                if (!\count($this->parser->getBlockStack())) {
                    throw new SyntaxError('Calling "parent" outside a block is forbidden.', $line, $this->parser->getStream()->getSourceContext());
                }

                if (!$this->parser->getParent() && !$this->parser->hasTraits()) {
                    throw new SyntaxError('Calling "parent" on a template that does not extend nor "use" another template is forbidden.', $line, $this->parser->getStream()->getSourceContext());
                }

                return new ParentExpression($this->parser->peekBlockStack(), $line);
            case 'block':
                $args = $this->parseArguments();
                if (\count($args) < 1) {
                    throw new SyntaxError('The "block" function takes one argument (the block name).', $line, $this->parser->getStream()->getSourceContext());
                }

                return new BlockReferenceExpression($args->getNode(0), \count($args) > 1 ? $args->getNode(1) : null, $line);
            case 'attribute':
                $args = $this->parseArguments();
                if (\count($args) < 2) {
                    throw new SyntaxError('The "attribute" function takes at least two arguments (the variable and the attributes).', $line, $this->parser->getStream()->getSourceContext());
                }

final class ExtendsTokenParser extends AbstractTokenParser
{
    public function parse(Token $token): Node
    {
        $stream = $this->parser->getStream();

        if ($this->parser->peekBlockStack()) {
            throw new SyntaxError('Cannot use "extend" in a block.', $token->getLine(), $stream->getSourceContext());
        } elseif (!$this->parser->isMainScope()) {
            throw new SyntaxError('Cannot use "extend" in a macro.', $token->getLine(), $stream->getSourceContext());
        }

        if (null !== $this->parser->getParent()) {
            throw new SyntaxError('Multiple extends tags are forbidden.', $token->getLine(), $stream->getSourceContext());
        }
        $this->parser->setParent($this->parser->getExpressionParser()->parseExpression());

        $stream->expect(/* Token::BLOCK_END_TYPE */ 3);