* Contrast the method above to:
 *
 * <ul>
 *   <li>methods whose return type is a type variable but which can never return {@code null},
 *       typically because the type forbids nullable type arguments: For example, {@code
 *       ImmutableList.get} returns {@code E}, but that value is never {@code null}. (Accordingly,
 *       {@code ImmutableList} is declared to forbid {@code ImmutableList<@Nullable String>}.)

  `What IS the fun?' said Alice.

  `Why, SHE,' said the Gryphon.  `It's all her fancy, that:  they
never executes nobody, you know.  Come on!'

  `Everybody says "come on!" here,' thought Alice, as she went
slowly after it:  `I never was so ordered about in all my life,
never!'

  They had not gone far before they saw the Mock Turtle in the
distance, sitting sad and lonely on a little ledge of rock, and,

 * Contrast the method above to:
 *
 * <ul>
 *   <li>methods whose return type is a type variable but which can never return {@code null},
 *       typically because the type forbids nullable type arguments: For example, {@code
 *       ImmutableList.get} returns {@code E}, but that value is never {@code null}. (Accordingly,
 *       {@code ImmutableList} is declared to forbid {@code ImmutableList<@Nullable String>}.)

 * Contrast the method above to:
 *
 * <ul>
 *   <li>methods whose return type is a type variable but which can never return {@code null},
 *       typically because the type forbids nullable type arguments: For example, {@code
 *       ImmutableList.get} returns {@code E}, but that value is never {@code null}. (Accordingly,
 *       {@code ImmutableList} is declared to forbid {@code ImmutableList<@Nullable String>}.)

 * Contrast the method above to:
 *
 * <ul>
 *   <li>methods whose return type is a type variable but which can never return {@code null},
 *       typically because the type forbids nullable type arguments: For example, {@code
 *       ImmutableList.get} returns {@code E}, but that value is never {@code null}. (Accordingly,
 *       {@code ImmutableList} is declared to forbid {@code ImmutableList<@Nullable String>}.)

 * Contrast the method above to:
 *
 * <ul>
 *   <li>methods whose return type is a type variable but which can never return {@code null},
 *       typically because the type forbids nullable type arguments: For example, {@code
 *       ImmutableList.get} returns {@code E}, but that value is never {@code null}. (Accordingly,
 *       {@code ImmutableList} is declared to forbid {@code ImmutableList<@Nullable String>}.)

 * Contrast the method above to:
 *
 * <ul>
 *   <li>methods whose return type is a type variable but which can never return {@code null},
 *       typically because the type forbids nullable type arguments: For example, {@code
 *       ImmutableList.get} returns {@code E}, but that value is never {@code null}. (Accordingly,
 *       {@code ImmutableList} is declared to forbid {@code ImmutableList<@Nullable String>}.)

 * Contrast the method above to:
 *
 * <ul>
 *   <li>methods whose return type is a type variable but which can never return {@code null},
 *       typically because the type forbids nullable type arguments: For example, {@code
 *       ImmutableList.get} returns {@code E}, but that value is never {@code null}. (Accordingly,
 *       {@code ImmutableList} is declared to forbid {@code ImmutableList<@Nullable String>}.)

    public Entry<K, V> belowSamplesLesser() {
      // should never reach here!
      throw new UnsupportedOperationException();
    }

    @Override
    public Entry<K, V> belowSamplesGreater() {
      // should never reach here!
      throw new UnsupportedOperationException();
    }

    @Override
    public Entry<K, V> aboveSamplesLesser() {
      // should never reach here!
      throw new UnsupportedOperationException();

  @Override
  final UnmodifiableIterator<Cell<R, C, V>> cellIterator() {
    throw new AssertionError("should never be called");
  }

  @Override
  final Spliterator<Cell<R, C, V>> cellSpliterator() {
    throw new AssertionError("should never be called");
  }

  @Override
  public ImmutableCollection<V> values() {
    return (ImmutableCollection<V>) super.values();
  }