code: function (mutation) {
          // for `set` and `unset` mutations the key to compact with is the `keyPath`
          var key = mutation[0];
  
          // convert `keyPath` to a string
          key = Array.isArray(key) ? key.join('.') : key;
  
          this.compacted[key] = mutation;
      }
docs: Records a mutation compacting existing mutations for the same key path.
  
  @private
  @param {MutationTracker~mutation} mutation