code: function(f, stop) {
           var sum, curr, i;
  
           i = 0;
           curr = f(0);
           sum = curr;
           return utils.repeat(sum, function() {
              i += 1;
              curr = f(i, curr);
              sum += curr;
              return sum;
           }, stop);
        }
docs: /* series
  - `f` is a function two arguments, `index` and `prev` (previous
  value).  It will be called starting at `index` 0.
  - `stop` is the same as in `repeat`.
  Add up the terms generated by `f` for index >= 0; stopping conditions
  are the same as for `repeat`.