code: func (d *doubleIter) remaining() remaining {
  	if d.from.current == nil && d.to.current == nil {
  		return noMoreNoders
  	}
  
  	if d.from.current == nil && d.to.current != nil {
  		return onlyToRemains
  	}
  
  	if d.from.current != nil && d.to.current == nil {
  		return onlyFromRemains
  	}
  
  	return bothHaveNodes
  }
docs: // Remaining returns if there are no more noders in the tree, if both
  // have noders or if one of them doesn't.