ImmutableTable.Builder<Character, Integer, String> builder = new ImmutableTable.Builder<>();

    StringHolder holder = new StringHolder();
    holder.string = "foo";
    Table.Cell<Character, Integer, String> mutableCell =
        new Tables.AbstractCell<Character, Integer, String>() {
          @Override
          public Character getRowKey() {
            return 'K';
          }

          @Override