pkg/flags/selective_string.go
func NewSelectiveStringValue(valids ...string) *SelectiveStringValue {
	vm := make(map[string]struct{})
	for _, v := range valids {
		vm[v] = struct{}{}
	}
	return &SelectiveStringValue{valids: vm, v: valids[0]}
}
func NewSelectiveStringValue(valids ...string) *SelectiveStringValue {
	vm := make(map[string]struct{})
	for _, v := range valids {
		vm[v] = struct{}{}
	}
	return &SelectiveStringValue{valids: vm, v: valids[0]}
}
// NewSelectiveStringValue creates a new string flag
// for which any one of the given strings is a valid value,
// and any other value is an error.
//
// valids[0] will be default value. Caller must be sure
// len(valids) != 0 or it will panic.