code: @Override
      public void undo(SubsetSolution solution) {
          // remove all added IDs
          solution.deselectAll(add);
          // add all removed IDs
          solution.selectAll(delete);
      }
docs: Undo this move after it has been successfully applied to the given subset solution,
  by removing all added IDs and adding all removed IDs. If the subset solution has been
  modified since successful application of this move, the result of this operation is
  undefined.
  
  @param solution solution to which the move has been successfully applied