code: func (t *TxStructure) HInc(key []byte, field []byte, step int64) (int64, error) {
  	if t.readWriter == nil {
  		return 0, errWriteOnSnapshot
  	}
  	base := int64(0)
  	err := t.updateHash(key, field, func(oldValue []byte) ([]byte, error) {
  		if oldValue != nil {
  			var err error
  			base, err = strconv.ParseInt(string(oldValue), 10, 64)
  			if err != nil {
  				return nil, errors.Trace(err)
  			}
  		}
  		base += step
  		return t.hashFieldIntegerVal(base), nil
  	})
  
  	return base, errors.Trace(err)
  }
docs: // HInc increments the integer value of a hash field, by step, returns
  // the value after the increment.