cmp/path.go
func (si SliceIndex) Key() int {
	if si.xkey != si.ykey {
		return -1
	}
	return si.xkey
}
func (si SliceIndex) Key() int {
	if si.xkey != si.ykey {
		return -1
	}
	return si.xkey
}
// Key is the index key; it may return -1 if in a split state