func (b counterStateBits) clearExtra() counterStateBits   { return b &^ stateExtra }
func (b counterStateBits) addExtra(n uint64) counterStateBits {
	const maxExtra = uint64(stateExtra) >> stateExtraShift // 0x1ffffffff
	x := b.extra()
	if x+n < x || x+n > maxExtra {
		x = maxExtra
	} else {
		x += n
	}
	return b.clearExtra() | counterStateBits(x)<<stateExtraShift
}