func (i *Int64Slice) Serialize() string {
	jsonBytes, _ := json.Marshal(i.slice)
	return fmt.Sprintf("%s%s", slPfx, string(jsonBytes))
}
Serialize allows Int64Slice to fulfill Serializer
package cli
import (
	"encoding/json"
	"flag"
	"fmt"
	"strconv"
	"strings"
)
type Int64Slice struct {
	slice      []int64
	hasBeenSet bool
}
func NewInt64Slice(defaults ...int64) *Int64Slice {
	return &Int64Slice{slice: append([]int64{}, defaults...)}
}
func (i *Int64Slice) Set(value string) error {
	if !i.hasBeenSet {
		i.slice = []int64{}
		i.hasBeenSet = true
	}
	if strings.HasPrefix(value, slPfx) {
		_ = json.Unmarshal([]byte(strings.Replace(value, slPfx, "", 1)), &i.slice)
		i.hasBeenSet = true
		return nil
	}
	tmp, err := strconv.ParseInt(value, 0, 64)
	if err != nil {
		return err
	}
	i.slice = append(i.slice, tmp)
	return nil
}
func (i *Int64Slice) String() string {
	return fmt.Sprintf("%#v", i.slice)
}