code: func (t *TxStructure) Inc(key []byte, step int64) (int64, error) {
  	if t.readWriter == nil {
  		return 0, errWriteOnSnapshot
  	}
  	ek := t.encodeStringDataKey(key)
  	// txn Inc will lock this key, so we don't lock it here.
  	n, err := kv.IncInt64(t.readWriter, ek, step)
  	if kv.ErrNotExist.Equal(err) {
  		err = nil
  	}
  	return n, errors.Trace(err)
  }
docs: // Inc increments the integer value of a key by step, returns
  // the value after the increment.