code: func (h *Holder) CreateIndex(name string, opt IndexOptions) (*Index, error) {
  	h.mu.Lock()
  	defer h.mu.Unlock()
  
  	// Ensure index doesn't already exist.
  	if h.indexes[name] != nil {
  		return nil, newConflictError(ErrIndexExists)
  	}
  	return h.createIndex(name, opt)
  }
docs: // CreateIndex creates an index.
  // An error is returned if the index already exists.