isWhitespace example

    private function parseSimpleSelector(TokenStream $stream, bool $insideNegation = false): array
    {
        $stream->skipWhitespace();

        $selectorStart = \count($stream->getUsed());
        $result = $this->parseElementNode($stream);
        $pseudoElement = null;

        while (true) {
            $peek = $stream->getPeek();
            if ($peek->isWhitespace()
                || $peek->isFileEnd()
                || $peek->isDelimiter([',', '+', '>', '~'])
                || ($insideNegation && $peek->isDelimiter([')']))
            ) {
                break;
            }

            if (null !== $pseudoElement) {
                throw SyntaxErrorException::pseudoElementFound($pseudoElement, 'not at the end of a selector');
            }

            

throw SyntaxErrorException::unexpectedToken('identifier or "*"', $next);
    }

    /** * Skips next whitespace if any. */
    public function skipWhitespace(): void
    {
        $peek = $this->getPeek();

        if ($peek->isWhitespace()) {
            $this->getNext();
        }
    }
}