assertFalse("Listener called more than once", wasCalled());
      called.set(true);
    }

    public void expectCall() {
      assertFalse("expectCall is already true", expectCall);
      expectCall = true;
    }

    public boolean wasCalled() {
      return called.get();
    }
  }

  public void testAllAsList() throws Exception {

      assertFalse("Listener called more than once", wasCalled());
      called.set(true);
    }

    public void expectCall() {
      assertFalse("expectCall is already true", expectCall);
      expectCall = true;
    }

    public boolean wasCalled() {
      return called.get();
    }
  }

  public void testAllAsList() throws Exception {

     * purposes, the "count" field, tracking the number of elements, serves as that volatile
     * variable ensuring visibility. This is convenient because this field needs to be read in many
     * read operations anyway:
     *
     * - All (unsynchronized) read operations must first read the "count" field, and should not look
     * at table entries if it is 0.
     *

   *
   * <p>If this {@code ClosingFuture} fails with an exception of the given type, the derived {@code
   * ClosingFuture} will be equivalent to the one returned by the function.
   *
   * <p>If this {@code ClosingFuture} succeeds or fails with a different exception type, the
   * function will not be called, and the derived {@code ClosingFuture} will be equivalent to this
   * one.
   *

     * purposes, the "count" field, tracking the number of elements, serves as that volatile
     * variable ensuring visibility. This is convenient because this field needs to be read in many
     * read operations anyway:
     *
     * - All (unsynchronized) read operations must first read the "count" field, and should not look
     * at table entries if it is 0.
     *

    return new IdentityHashMap<>();
  }

  /**
   * Computes the difference between two maps. This difference is an immutable snapshot of the state
   * of the maps at the time this method is called. It will never change, even if the maps change at
   * a later time.
   *
   * <p>Since this method uses {@code HashMap} instances internally, the keys of the supplied maps