* A builder for creating immutable table instances, especially {@code public static final} tables
   * ("constant tables"). Example:
   *
   * <pre>{@code
   * static final ImmutableTable<Integer, Character, String> SPREADSHEET =
   *     new ImmutableTable.Builder<Integer, Character, String>()
   *         .put(1, 'A', "foo")
   *         .put(1, 'B', "bar")
   *         .put(2, 'A', "baz")
   *         .buildOrThrow();
   * }</pre>