code: func (c *MockClient) Set(item *Item) MutateResponse {
  	c.mutex.Lock()
  	defer c.mutex.Unlock()
  
  	return c.setHelper(item)
  }
docs: // This sets a single entry into memcache.  If the item's data version id
  // (aka CAS) is nonzero, the set operation can only succeed if the item
  // exists in memcache and has a same data version id.