I would like to superimpose some clips onto the main Video
Track. However, this will mean that the clip on the A
track will totally obscure the video on B ounless I set
Opacy. Since the overlay clip has a black background, I
would like to set black to be alpha, ie. transparent.

How can I do this? I tried the content CD's explosion4.avi
but it didn't work, even when I changed the properties...