code: func (d *doubleIter) stepBoth() (err error) {
  	if d.from.current, err = d.from.iter.Step(); turnEOFIntoNil(err) != nil {
  		return err
  	}
  	if d.to.current, err = d.to.iter.Step(); turnEOFIntoNil(err) != nil {
  		return err
  	}
  	return nil
  }
docs: // StepBoth makes d advance to the next noder in both merkletries,
  // getting deeper into directories if that is the case.