docs: function reduce(iterable, fn, seed) {
    if ((seed === undefined) || (seed === Nill)) {
      [seed, iterable] = advance(iterable, seed);
      if (seed === Nill)
        return undefined;
    } // if ...
    let total = seed;
    for (const v of iterable)
      total = fn(total, v);
    return total;
  }