code: function (obj, f) {
        var r = {};
        each(obj, function (x, i) {
          var tuple = f(x, i, obj);
          r[tuple.k] = tuple.v;
        });
        return r;
      }
docs: tupleMap :: (JsObj(k, v), (v, k, JsObj(k, v) -> { k: x, v: y })) -> JsObj(x, y)