[Robert Arnold]

Just learned how to do this from another user on this forum!

1. Right-click in the node area and choose Add Alpha Output. A new gray rectangle will appear at the right.
2. Add a serial node and drag its matte output (the blue triangle) to the Alpha Output rectangle.
3. Use the Output Gain on the Key tab to control the opacity of the layer.

This is a huge help. Thanks to Brandon Thomas.