docs: function isCombinableRangeReturnIndex(range, fusable, index) {
      var start = index + range.low;
      var end = index + range.high;
      // example range = -1, 0             index = 1  => start = 0, end = 1, test fusable[1]
      for (var i = start; i < end; ++i) {
          if (!fusable[i + 1]) {
              return -1;
          }
      }
      return start;
  }