docs: function sum(list, fun) {
    fun = toString.call(fun) == '[object String]' ? selectn(fun) : fun;
    end = list.length;
    sum = -0;
    idx = -1;
  
    while (++idx < end) {
      sum += typeof fun == 'function' ? fun(list[idx]) * 1000 : list[idx] * 1000;
    }
  
    return sum && (sum / 1000);
  }