requireNonNull(impl);
      requireNonNull(other.impl);
      /*
       * For discussion of requireNonNull, see the comment on the field.
       *
       * (And I don't believe there's any situation in which we call x.combine(y) when x is a plain
       * ImmutableSet.Builder but y is an ImmutableSortedSet.Builder (or vice versa). Certainly

        int hash = map.hash(key);
        Object value = new Object();

        ReferenceEntry<Object, Object> entry = createDummyEntry(key, hash, value, null);
        // must recordRead for drainRecencyQueue to believe this entry is live
        segment.recordWrite(entry, 1, map.ticker.read());
        writeOrder.add(entry);
        readOrder.add(entry);
      }

        int hash = map.hash(key);
        Object value = new Object();

        ReferenceEntry<Object, Object> entry = createDummyEntry(key, hash, value, null);
        // must recordRead for drainRecencyQueue to believe this entry is live
        segment.recordWrite(entry, 1, map.ticker.read());
        writeOrder.add(entry);
        readOrder.add(entry);
      }

   * {@link TreeSet} or the {@link Map#keySet} of an {@code IdentityHashMap}.
   *
   * <p><b>Note:</b> The returned view performs slightly better when {@code set1} is the smaller of
   * the two sets. If you have reason to believe one of your sets will generally be smaller than the
   * other, pass it first. Unfortunately, since this method sets the generic type of the returned

    if (cause instanceof Error) {
      throw new ExecutionError((Error) cause);
    }
    /*
     * It's an Exception. (Or it's a non-Error, non-Exception Throwable. From my survey of such
     * classes, I believe that most users intended to extend Exception, so we'll treat it like an
     * Exception.)
     */
    throw new UncheckedExecutionException(cause);
  }

  /*

              <extraJvmArgs>-Dgwt.watchFileChanges=false</extraJvmArgs>
            </configuration>
          </execution>
          <!--
            GWT issues some scary warnings while running tests, but I believe they are harmless.
            Let's take the warnings for one module, c.g.c.base.testModule, as an example:

              [INFO] Validating units: