code: protected List<Row<I>> selectClosingRows(List<List<Row<I>>> unclosed) {
          List<Row<I>> closingRows = new ArrayList<>(unclosed.size());
  
          for (List<Row<I>> rowList : unclosed) {
              closingRows.add(rowList.get(0));
          }
  
          return closingRows;
      }
docs: This method selects a set of rows to use for closing the table. It receives as input a list of row lists, such
  that each (inner) list contains long prefix rows with (currently) identical contents, which have no matching
  short prefix row. The outer list is the list of all those equivalence classes.
  
  @param unclosed
  a list of equivalence classes of unclosed rows.
  
  @return a list containing a representative row from each class to move to the short prefix part.