func (b *FixedSizeRingBuf) LegalPos() (a0, aLast, b0, bLast int) {
	a0 = -1
	aLast = -1
	b0 = -1
	bLast = -1
	if b.Readable == 0 {
		return
	}
	a0 = b.Beg
	last := b.Beg + b.Readable - 1
	if last < b.N {
		aLast = last
		return
	}
	aLast = b.N - 1
	b0 = 0
	bLast = last % b.N
	return
}
func (b *FixedSizeRingBuf) LegalPos() (a0, aLast, b0, bLast int) {
	a0 = -1
	aLast = -1
	b0 = -1
	bLast = -1
	if b.Readable == 0 {
		return
	}
	a0 = b.Beg
	last := b.Beg + b.Readable - 1
	if last < b.N {
		aLast = last
		return
	}
	aLast = b.N - 1
	b0 = 0
	bLast = last % b.N
	return
}
// LegalPos returns the legal index positions,
// [a0,aLast] and [b0,bLast] inclusive, where the
// [a0,aLast] holds the first FIFO ordered segment,
// and the [b0,bLast] holds the second ordered segment,
// if any.
// A position of -1 means the segment is not used,
// perhaps because b.Readable is zero, or because
// the second segment [b0,bLast] is not in use (when
// everything fits in the first [a0,aLast] segment).
//