* {@linkplain Object#equals equal} to the original, nor is it required to return even an object
   * of the same class. For example, if sublists of {@code MyList} instances were serializable,
   * those sublists might implement a private {@code MySubList} type but serialize as a plain {@code
   * MyList} to save space. So long as {@code MyList} has all the public supertypes of {@code

        fail(
            "contents were not equal and in the same order: "
                + "expected = "
                + expected
                + ", actual = "
                + actual);
      }
    }

    if (expectedIter.hasNext() || actualIter.hasNext()) {
      // actual either had too few or too many elements
      fail(
          "contents were not equal and in the same order: "

  public static <T extends @Nullable Object> Collector<T, ?, T> onlyElement() {
    return (Collector) ONLY_ELEMENT;
  }

  /**
   * This atrocity is here to let us report several of the elements in the stream if there were more
   * than one, not just two.
   */
  @SuppressWarnings("EmptyList") // ImmutableList doesn't support nullable element types
  private static final class ToOptionalState {
    static final int MAX_EXTRAS = 4;

   * analysis that that means "plain {@code T}" rather than the inferred type {@code @Nullable T}.
   * (And even if annotations on local variables were permitted as an optional hint, no annotation
   * would be the right tool for the job here: {@code @Nullable} is the annotation that we're trying

   * with {@code FeatureSpecificTestSuiteBuilder.suppressing()} until <a
   * href="https://bugs.openjdk.org/browse/JDK-6409434">JDK-6409434</a> is fixed. It's unclear
   * whether nulls were to be permitted or forbidden, but presumably the eventual fix will be to
   * permit them, as it seems more likely that code would depend on that behavior than on the other.

  private static final long K1 = 0x0f0e0d0c0b0a0908L;
  private static final HashFunction SIP_WITH_KEY = Hashing.sipHash24(K0, K1);
  private static final HashFunction SIP_WITHOUT_KEY = Hashing.sipHash24();

  // These constants were originally ported from https://www.131002.net/siphash/siphash24.c. See:
  // https://github.com/nahi/siphash-java-inline/blob/master/src/test/java/org/jruby/util/SipHashInlineTest.java
  private static final long[] EXPECTED =

   * with {@code FeatureSpecificTestSuiteBuilder.suppressing()} until <a
   * href="https://bugs.openjdk.org/browse/JDK-6409434">JDK-6409434</a> is fixed. It's unclear
   * whether nulls were to be permitted or forbidden, but presumably the eventual fix will be to
   * permit them, as it seems more likely that code would depend on that behavior than on the other.

  public static <T extends @Nullable Object> Collector<T, ?, T> onlyElement() {
    return (Collector) ONLY_ELEMENT;
  }

  /**
   * This atrocity is here to let us report several of the elements in the stream if there were more
   * than one, not just two.
   */
  @SuppressWarnings("EmptyList") // ImmutableList doesn't support nullable element types
  private static final class ToOptionalState {
    static final int MAX_EXTRAS = 4;

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      represent, as a whole, an original work of authorship. For the purposes
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      the Work and Derivative Works thereof.

      "Contribution" shall mean any work of authorship, including