var ErrNotImplemented = errors.New("not implemented")

// ReadFrom implements support for calling internal io.ReaderFrom implementations
// returns an error if the underlying ResponseWriter does not implement io.ReaderFrom
func (lrw *ResponseRecorder) ReadFrom(r io.Reader) (int64, error) {
	if lrw.ReaderFrom != nil {
		n, err := lrw.ReaderFrom.ReadFrom(r)
		lrw.bytesWritten += int(n)
		return n, err
	}

pkg net/url, method (*URL) EscapedFragment() string
pkg net/url, method (*URL) Redacted() string
pkg net/url, type URL struct, RawFragment string
pkg os, method (*File) ReadFrom(io.Reader) (int64, error)
pkg os, var ErrDeadlineExceeded error
pkg regexp, method (*Regexp) SubexpIndex(string) int
pkg strconv, func FormatComplex(complex128, uint8, int, int) string

	type nErr struct {
		n   int64
		err error
	}
	type justReader struct {
		io.Reader
	}
	type justWriter struct {
		io.Writer
	}
	discard := justWriter{io.Discard} // hide ReadFrom

	var with, withOut nErr
	with.n, with.err = io.Copy(discard, NewReader(nil))
	withOut.n, withOut.err = io.Copy(discard, justReader{NewReader(nil)})
	if with != withOut {