docs: function NDDBIndex(idx, nddb) {
          // The name of the index.
          this.idx = idx;
          // Reference to the whole nddb database.
          this.nddb = nddb;
          // Map indexed-item to a position in the original database.
          this.resolve = {};
          // List of all keys in `resolve` object.
          this.keys = [];
          // Map indexed-item to a position in `keys` array (for fast deletion).
          this.resolveKeys = {};
      }