code: function(key) {
          this._removeRemove(this.maps.remove, key);
          var map = this._getOp(this.maps.modify);
          if (map) {
              return map;
          } else {
              map = new MapOperation();
              this.maps.modify.push({key: key, map: map});
              return map;
          }
      }
docs: Access/create a map inside the current one.
  This adds/accesses a nested map and returns a reference to another
  MapOperation that applies to it. You can then modify the components of
  that map;
  
  mapOp.map('inner_map')
  .incrementCounter('counter_1', 50)
  .addToSet('set_2', 'set_value_2');
  @method map
  @param {String} key the key for the nested map in the current map.
  @chainable