if end < 0 {
			return nDst, nSrc, errs[-end]
		}
		nDst = len(dst) - len(rb.out)
		nSrc = end

		// Next quickSpan.
		end = rb.nsrc
		eof := atEOF
		if n := nSrc + len(dst) - nDst; n < end {
			err = transform.ErrShortDst
			end = n
			eof = false
		}
		end, ok := rb.f.quickSpan(rb.src, nSrc, end, eof)
		n := copy(dst[nDst:], rb.src.bytes[nSrc:end])
		nSrc += n
		nDst += n
		if ok {

func (f Form) AppendString(out []byte, src string) []byte {
	return f.doAppend(out, inputString(src), len(src))
}

// QuickSpan returns a boundary n such that b[0:n] == f(b[0:n]).
// It is not guaranteed to return the largest such n.
func (f Form) QuickSpan(b []byte) int {
	n, _ := formTable[f].quickSpan(inputBytes(b), 0, len(b), true)
	return n
}

// Span implements transform.SpanningTransformer. It returns a boundary n such

		n := copy(src, s[pSrc:])
		nDst, nSrc, err = t.Transform(dst, src[:n], pSrc+n == len(s))
		pDst += nDst
		pSrc += nSrc

		// TODO:  let transformers implement an optional Spanner interface, akin
		// to norm's QuickSpan. This would even allow us to avoid any allocation.
		if !bytes.Equal(dst[:nDst], src[:nSrc]) {
			break
		}
		pPrefix = pSrc
		if err == ErrShortDst {