}
    }

    @Override
    public NavigableMap<K, V> headMap(K toKey, boolean inclusive) {
      synchronized (mutex) {
        return navigableMap(delegate().headMap(toKey, inclusive), mutex);
      }
    }

    @Override
    public SortedMap<K, V> headMap(K toKey) {
      return headMap(toKey, false);
    }

    @Override

      }
    }

    @Override
    public NavigableMap<K, V> headMap(K toKey, boolean inclusive) {
      synchronized (mutex) {
        return navigableMap(delegate().headMap(toKey, inclusive), mutex);
      }
    }

    @Override
    public SortedMap<K, V> headMap(K toKey) {
      return headMap(toKey, false);
    }

    @Override

      if (wrapper.getCause() instanceof Error) {
        throw new ExecutionError((Error) wrapper.getCause());
      }
      /*
       * 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(wrapper.getCause());
    }