cmp/options.go
func FilterPath(f func(Path) bool, opt Option) Option {
	if f == nil {
		panic("invalid path filter function")
	}
	if opt := normalizeOption(opt); opt != nil {
		return &pathFilter{fnc: f, opt: opt}
	}
	return nil
}
func FilterPath(f func(Path) bool, opt Option) Option {
	if f == nil {
		panic("invalid path filter function")
	}
	if opt := normalizeOption(opt); opt != nil {
		return &pathFilter{fnc: f, opt: opt}
	}
	return nil
}
// FilterPath returns a new Option where opt is only evaluated if filter f
// returns true for the current Path in the value tree.
//
// This filter is called even if a slice element or map entry is missing and
// provides an opportunity to ignore such cases. The filter function must be
// symmetric such that the filter result is identical regardless of whether the
// missing value is from x or y.
//
// The option passed in may be an Ignore, Transformer, Comparer, Options, or
// a previously filtered Option.