code: func (svmMap *serviceVMMap) deleteID(id string) error {
  	svmMap.Lock()
  	defer svmMap.Unlock()
  	svm, ok := svmMap.svms[id]
  	if !ok {
  		return errVMUnknown
  	}
  	if svm.refCount != 0 {
  		return errVMStillHasReference
  	}
  	delete(svmMap.svms, id)
  	return nil
  }
docs: // deleteID deletes the given ID from the map. If the refcount is not 0 or the
  // VM does not exist, then this function returns an error.