docs: function reduce(accumulator, seed) {
      var hasSeed = false;
      // providing a seed of `undefined` *should* be valid and trigger
      // hasSeed! so don't use `seed !== undefined` checks!
      // For this reason, we have to check it here at the original call site
      // otherwise inside Operator/Subscriber we won't know if `undefined`
      // means they didn't provide anything or if they literally provided `undefined`
      if (arguments.length >= 2) {
          hasSeed = true;
      }
      return this.lift(new ReduceOperator(accumulator, seed, hasSeed));
  }