code: function mergeSummaryObjects() {
      var ret = blankSummary(),
        args = Array.prototype.slice.call(arguments),
        keys = ['lines', 'statements', 'branches', 'functions'],
        increment = function (obj) {
          if (obj) {
            keys.forEach(function (key) {
              ret[key].total += obj[key].total;
              ret[key].covered += obj[key].covered;
              ret[key].skipped += obj[key].skipped;
            });
          }
        };
      args.forEach(function (arg) {
        increment(arg);
      });
      keys.forEach(function (key) {
        ret[key].pct = percent(ret[key].covered, ret[key].total);
      });
  
      return ret;
    }
docs: merges multiple summary metrics objects by summing up the `totals` and
  `covered` fields and recomputing the percentages. This function is generic
  and can accept any number of arguments.
  
  @method mergeSummaryObjects
  @static
  @param {Object} summary... multiple summary metrics objects
  @return {Object} the merged summary metrics