code: function arrayUpdate(mutate, at, v, arr) {
      var out = arr;
      if (!mutate) {
          var len = arr.length;
          out = new Array(len);
          for (var i = 0; i < len; ++i) {
              out[i] = arr[i];
          }
      }
      out[at] = v;
      return out;
  }
docs: /* Array Ops
  **************************************************************************** 
  Set a value in an array.
  
  @param mutate Should the input array be mutated?
  @param at Index to change.
  @param v New value
  @param arr Array.