func (b *Reader) Size() int { return len(b.buf) }

// Reset discards any buffered data, resets all state, and switches
// the buffered reader to read from r.
// Calling Reset on the zero value of [Reader] initializes the internal buffer
// to the default size.
// Calling b.Reset(b) (that is, resetting a [Reader] to itself) does nothing.
func (b *Reader) Reset(r io.Reader) {
	// If a Reader r is passed to NewReader, NewReader will return r.