code: function evalIndex(list, index, default_) {
          if (index && ((index.type !== "Dimension") ||
              !index.unit.isEmpty())) throw {
                  type:    "Argument",
                  message: "unexpected index `" + index.toCSS() + "`"
          };
  
          index = (index === undefined)
              ? default_ : index.value;
          return index + ((index < 1)
              ? list.value.length : -1);
      }
docs: ........................................................