func (b *FixedSizeRingBuf) Advance(n int) {
	if n <= 0 {
		return
	}
	if n > b.Readable {
		n = b.Readable
	}
	b.Readable -= n
	b.Beg = (b.Beg + n) % b.N
}
func (b *FixedSizeRingBuf) Advance(n int) {
	if n <= 0 {
		return
	}
	if n > b.Readable {
		n = b.Readable
	}
	b.Readable -= n
	b.Beg = (b.Beg + n) % b.N
}
// Advance(): non-standard, but better than Next(),
// because we don't have to unwrap our buffer and pay the cpu time
// for the copy that unwrapping may need.
// Useful in conjuction/after ReadWithoutAdvance() above.