code: func IncInt64(rm RetrieverMutator, k Key, step int64) (int64, error) {
  	val, err := rm.Get(k)
  	if IsErrNotFound(err) {
  		err = rm.Set(k, []byte(strconv.FormatInt(step, 10)))
  		if err != nil {
  			return 0, err
  		}
  		return step, nil
  	}
  	if err != nil {
  		return 0, err
  	}
  
  	intVal, err := strconv.ParseInt(string(val), 10, 0)
  	if err != nil {
  		return 0, errors.Trace(err)
  	}
  
  	intVal += step
  	err = rm.Set(k, []byte(strconv.FormatInt(intVal, 10)))
  	if err != nil {
  		return 0, err
  	}
  	return intVal, nil
  }
docs: // IncInt64 increases the value for key k in kv store by step.