copy/manifest.go
func (os *orderedSet) append(s string) {
	if _, ok := os.included[s]; !ok {
		os.list = append(os.list, s)
		os.included[s] = struct{}{}
	}
}
func (os *orderedSet) append(s string) {
	if _, ok := os.included[s]; !ok {
		os.list = append(os.list, s)
		os.included[s] = struct{}{}
	}
}
// append adds s to the end of os, only if it is not included already.