code: @GwtIncompatible // ImmutableSortedSet.indexOf
    // TODO(cpovirk): consider manual binary search under GWT to preserve O(log N) lookup
    @Override
    public int indexOf(@Nullable Object target) {
      int index = delegateCollection().indexOf(target);
  
      // TODO(kevinb): reconsider if it's really worth making feeble attempts at
      // sanity for inconsistent comparators.
  
      // The equals() check is needed when the comparator isn't compatible with
      // equals().
      return (index >= 0 && get(index).equals(target)) ? index : -1;
    }
docs: Override indexOf() and lastIndexOf() to be O(log N) instead of O(N).