code: func (s *MockStore) Put(key string, value []byte, options *store.WriteOptions) error {
  	mData := s.db[key]
  	if mData == nil {
  		mData = &MockData{value, 0}
  	}
  	mData.Index = mData.Index + 1
  	s.db[key] = mData
  	return nil
  }
docs: // Put a value at "key"