pkg/flags/selective_string.go
func NewSelectiveStringsValue(valids ...string) *SelectiveStringsValue {
	vm := make(map[string]struct{})
	for _, v := range valids {
		vm[v] = struct{}{}
	}
	return &SelectiveStringsValue{valids: vm, vs: []string{}}
}
func NewSelectiveStringsValue(valids ...string) *SelectiveStringsValue {
	vm := make(map[string]struct{})
	for _, v := range valids {
		vm[v] = struct{}{}
	}
	return &SelectiveStringsValue{valids: vm, vs: []string{}}
}
// NewSelectiveStringsValue creates a new string slice flag
// for which any one of the given strings is a valid value,
// and any other value is an error.