clientv3/compare.go
func mustInt64(val interface{}) int64 {
	if v, ok := val.(int64); ok {
		return v
	}
	if v, ok := val.(int); ok {
		return int64(v)
	}
	panic("bad value")
}
func mustInt64(val interface{}) int64 {
	if v, ok := val.(int64); ok {
		return v
	}
	if v, ok := val.(int); ok {
		return int64(v)
	}
	panic("bad value")
}
// mustInt64 panics if val isn't an int or int64. It returns an int64 otherwise.