code: function (plus, minus) {
      var out = {}, i;
  
      for (i in plus) {
        if (plus.hasOwnProperty(i) && plus[i] !== minus[i]) {
          out[i] = plus[i];
        }
      }
  
      return out;
    }
docs: Remove all elements of minus from plus.  Does not modify plus.
  @private