func (f *FixedSizeRingBuf) Prevpos(from int) int {
	if from >= f.N || from < 0 {
		return -2
	}
	if f.Readable == 0 {
		return -1
	}
	if from == f.Beg {
		return -1
	}
	a0, a1, b0, b1 := f.LegalPos()
	switch {
	case from == a0:
		return -1
	case from > a0 && from <= a1:
		return from - 1
	case from == b0:
		return a1
	case from > b0 && from <= b1:
		return from - 1
	}
	return -1
}
func (f *FixedSizeRingBuf) Prevpos(from int) int {
	if from >= f.N || from < 0 {
		return -2
	}
	if f.Readable == 0 {
		return -1
	}
	if from == f.Beg {
		return -1
	}
	a0, a1, b0, b1 := f.LegalPos()
	switch {
	case from == a0:
		return -1
	case from > a0 && from <= a1:
		return from - 1
	case from == b0:
		return a1
	case from > b0 && from <= b1:
		return from - 1
	}
	return -1
}
// Prevpos returns the index of the element before
// from, or -1 if no more and from is the
// first in the ring. Returns -2 on bad
// from position.