bf := b[:min(int64(len(b)), dataStart-sw.pos)]
			nf, err = zeroWriter{}.Write(bf)
		} else { // In a data fragment
			bf := b[:min(int64(len(b)), dataEnd-sw.pos)]
			nf, err = sw.fw.Write(bf)
		}
		b = b[nf:]
		sw.pos += int64(nf)
		if sw.pos >= dataEnd && len(sw.sp) > 1 {
			sw.sp = sw.sp[1:] // Ensure last fragment always remains
		}
	}

	n = len(b0) - len(b)
	switch {
	case err == ErrWriteTooLong:

// why the write is short.
func (b *Writer) WriteString(s string) (int, error) {
	var sw io.StringWriter
	tryStringWriter := true

	nn := 0
	for len(s) > b.Available() && b.err == nil {
		var n int
		if b.Buffered() == 0 && sw == nil && tryStringWriter {
			// Check at most once whether b.wr is a StringWriter.
			sw, tryStringWriter = b.wr.(io.StringWriter)
		}
		if b.Buffered() == 0 && tryStringWriter {