code: func SplitString(b []byte) (content, rest []byte, err error) {
  	k, content, rest, err := Split(b)
  	if err != nil {
  		return nil, b, err
  	}
  	if k == List {
  		return nil, b, ErrExpectedString
  	}
  	return content, rest, nil
  }
docs: // SplitString splits b into the content of an RLP string
  // and any remaining bytes after the string.