code: func (h *Holder) CreateIndexIfNotExists(name string, opt IndexOptions) (*Index, error) {
  	h.mu.Lock()
  	defer h.mu.Unlock()
  
  	// Find index in cache first.
  	if index := h.indexes[name]; index != nil {
  		return index, nil
  	}
  
  	return h.createIndex(name, opt)
  }
docs: // CreateIndexIfNotExists returns an index by name.
  // The index is created if it does not already exist.