code: func (ot *orderedTree) InsertAtDimension(dimension uint64,
  	index, number int64) (Entries, Entries) {
  
  	// TODO: perhaps return an error here?
  	if dimension > ot.dimensions || number == 0 {
  		return nil, nil
  	}
  
  	modified := make(Entries, 0, 100)
  	deleted := make(Entries, 0, 100)
  
  	ot.top.insert(dimension, 1, ot.dimensions,
  		index, number, &modified, &deleted,
  	)
  
  	ot.number -= uint64(len(deleted))
  
  	return modified, deleted
  }
docs: // InsertAtDimension will increment items at and above the given index
  // by the number provided.  Provide a negative number to to decrement.
  // Returned are two lists.  The first list is a list of entries that
  // were moved.  The second is a list entries that were deleted.  These
  // lists are exclusive.