code: function (y, d) {
  
          var self = this;
          y = makeCompareFunction(y, d);
          return extend(function (a, b) {
  
              return self(a, b) || y(a, b);
          });
      }
docs: /* adds a secondary compare function to the target function (`this` context)
  which is applied in case the first one returns 0 (equal)
  returns a new compare function, which has a `thenBy` method as well