}

  public void testColumnKeySet_empty() {
    table = create();
    assertThat(table.columnKeySet().toString()).isEqualTo("[]");
  }

  public void testColumnKeySet_oneRow() {
    table = create("a", 2, 'X', "a", 1, 'X');
    assertThat(table.columnKeySet().toString()).isEqualTo("[1, 2]");
  }

  public void testColumnKeySet_oneColumn() {