code: function tb(func, opt) {
          /* should get value false for the first call. This can be done by calling the 
          exported function, or the firstBy property on it (for es6 module compatibility)
          */
          var x = (typeof(this) == "function" && !this.firstBy) ? this : false;
          var y = makeCompareFunction(func, opt);
          var f = x ? function(a, b) {
                          return x(a,b) || y(a,b);
                      }
                    : y;
          f.thenBy = tb;
          return f;
      }
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