func (b *AtomicFixedSizeRingBuf) unatomic_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 *AtomicFixedSizeRingBuf) unatomic_advance(n int) {
	if n <= 0 {
		return
	}
	if n > b.readable {
		n = b.readable
	}
	b.readable -= n
	b.Beg = (b.Beg + n) % b.N
}
// unatomic_advance(): private implementation of Advance() without
// the locks. See Advance() above for description.
// Necessary so that other methods that already hold
// locks can advance, and there are no recursive mutexes
// in Go.