* guarantee behavior in that case, anyway, and the current behavior is likely undesirable.
         * So that's either a reason to feel free to change it or a reason to not bother thinking
         * further about this.
         */
        V rightValue = uncheckedCastNullableTToT(onlyOnRight.remove(leftKey));

  }

  /**
   * Attempts to cancel execution of this step. This attempt will fail if the step has already
   * completed, has already been cancelled, or could not be cancelled for some other reason. If
   * successful, and this step has not started when {@code cancel} is called, this step should never
   * run.
   *
   * <p>If successful, causes the objects captured by this step (if already started) and its input