code: func (d *doubleIter) nextBoth() error {
  	if err := d.nextFrom(); err != nil {
  		return err
  	}
  	if err := d.nextTo(); err != nil {
  		return err
  	}
  
  	return nil
  }
docs: // NextBoth makes d advance to the next noder in both merkletries.  If
  // any of them is a directory, it skips its contents.