code: private function decodeLiteral(Context $context, $literal, $value)
      {
          $count = strlen($literal);
          if ($context->getSequence($count) !== $literal) {
              $current = $context->current();
              throw $context->createException("Unexpected character found ('{$current}')");
          }
          $this->result = $value;
          $context->skip($count);
      }
docs: リテラル null, true, false をデコードします.
  
  @param Context $context
  @param string  $literal
  @param mixed   $value