code: @CanIgnoreReturnValue
    public <V> V call(Callable<V> c) throws Exception {
      Context previous = attach();
      try {
        return c.call();
      } finally {
        detach(previous);
      }
    }
docs: Immediately call a {@link Callable} with this context as the {@link #current} context.
  @param c {@link Callable} to call.
  @return result of call.