code: public List<CellFiller> cell(final Symbol left, final Symbol right) {
      final List<CellFiller> cell = table.get(left, right);
      if (cell != null) {
        return cell;
      } else {
        return ImmutableList.of();
      }
    }
docs: A list of all the entries occupying the cell {@code (left, right)} of this confusion matrix.