* PeekingIterator#remove()}.
   *
   * <p>Note: If the given iterator is already a {@code PeekingIterator}, it <i>might</i> be
   * returned to the caller, although this is neither guaranteed to occur nor required to be
   * consistent. For example, this method <i>might</i> choose to pass through recognized

 *
 * <p>If the precondition is not met, the {@code Preconditions} method throws an unchecked exception
 * of a specified type, which helps the method in which the exception was thrown communicate that
 * its caller has made a mistake. This allows constructs such as
 *
 * <pre>{@code
 * public static double sqrt(double value) {
 *   if (value < 0) {
 *     throw new IllegalArgumentException("input is negative: " + value);
 *   }

    try {
      cache.getUnchecked("bar");
      fail();
    } catch (UncheckedExecutionException expected) {
    }
    assertEquals(2, callCount.get());
  }

  /**
   * Test-helper method that performs {@code nThreads} concurrent calls to {@code cache.get(key)} or
   * {@code cache.getUnchecked(key)}, and returns a List containing each of the results. The result

  private static final long SPIN_THRESHOLD_NANOS = 1000L;

  private static final AtomicHelper ATOMIC_HELPER;

  static {
    AtomicHelper helper;
    Throwable thrownUnsafeFailure = null;
    Throwable thrownAtomicReferenceFieldUpdaterFailure = null;

    try {
      helper = new UnsafeAtomicHelper();
    } catch (Exception | Error unsafeFailure) { // sneaky checked exception

    try {
      cache.getUnchecked("bar");
      fail();
    } catch (UncheckedExecutionException expected) {
    }
    assertEquals(2, callCount.get());
  }

  /**
   * Test-helper method that performs {@code nThreads} concurrent calls to {@code cache.get(key)} or
   * {@code cache.getUnchecked(key)}, and returns a List containing each of the results. The result

    static final class Helper<K, V>
        implements InternalEntryHelper<
            K, V, StrongKeyStrongValueEntry<K, V>, StrongKeyStrongValueSegment<K, V>> {
      private static final Helper<?, ?> INSTANCE = new Helper<>();

      @SuppressWarnings("unchecked")
      static <K, V> Helper<K, V> instance() {
        return (Helper<K, V>) INSTANCE;
      }

      @Override

        @Override
        public String toString() {
          return description;
        }
      };
    }
  }

  /**
   * Helper method for {@link #precomputedInternal} that doesn't test if the negation is cheaper.
   */
  @GwtIncompatible // SmallCharMatcher
  private static CharMatcher precomputedPositive(

  //    visible to done(). This is because there is no happens before edge between the write and a
  //    (hypothetical) unsafe read by our caller. Note: adding 'volatile' does not fix this issue,
  //    it would just add an edge such that if done() observed non-null, then it would also

 *
 * <p>If the precondition is not met, the {@code Preconditions} method throws an unchecked exception
 * of a specified type, which helps the method in which the exception was thrown communicate that
 * its caller has made a mistake. This allows constructs such as
 *
 * <pre>{@code
 * public static double sqrt(double value) {
 *   if (value < 0) {
 *     throw new IllegalArgumentException("input is negative: " + value);
 *   }

        @Override
        public String toString() {
          return description;
        }
      };
    }
  }

  /**
   * Helper method for {@link #precomputedInternal} that doesn't test if the negation is cheaper.
   */
  @GwtIncompatible // SmallCharMatcher
  private static CharMatcher precomputedPositive(