code: func (xft *XFastTrie) Exists(key uint64) bool {
  	// the bottom hashmap of the trie has every entry
  	// in it.
  	_, ok := xft.layers[xft.bits-1][key]
  	return ok
  }
docs: // Exists returns a bool indicating if the provided
  // key exists in the trie.  This is an O(1) operation.