code: @Override
      public void undo(SubsetSolution solution) {
          // re-add deleted ID
          solution.select(delete);
          // remove newly added ID
          solution.deselect(add);
      }
docs: Undo this swap move after it has been successfully applied to the given subset solution,
  by removing the newly added ID and re-adding the deleted ID.
  
  @param solution solution to which the move has been applied