code: function (item) {
          // do not proceed on empty param, but empty strings are in fact valid.
          if (_.isNil(item) || _.isNaN(item)) { return null; }
  
          var indexer = this._postman_listIndexKey,
              existing = this.one(item[indexer]);
  
          if (existing) {
              if (!_.isFunction(existing.update)) {
                  throw new Error('collection: unable to upsert into a list of Type that does not support .update()');
              }
              existing.update(item);
              return false;
          }
  
          // since there is no existing item, just add a new one
          this.add(item);
          return true; // indicate added
      }
docs: Add an item or update an existing item
  
  @param {PropertyList~Type} item
  
  @returns {?Boolean}