func (pa Path) Index(i int) PathStep {
	if i < 0 {
		i = len(pa) + i
	}
	if i < 0 || i >= len(pa) {
		return pathStep{}
	}
	return pa[i]
}
func (pa Path) Index(i int) PathStep {
	if i < 0 {
		i = len(pa) + i
	}
	if i < 0 || i >= len(pa) {
		return pathStep{}
	}
	return pa[i]
}
// Index returns the ith step in the Path and supports negative indexing.
// A negative index starts counting from the tail of the Path such that -1
// refers to the last step, -2 refers to the second-to-last step, and so on.
// If index is invalid, this returns a non-nil PathStep that reports a nil Type.