Subtracting Channels

Hi, new to Fusion, and currently playing with 8 Beta on OSX.

So I was mucking around with some colour blocks in a mograph style animation, and getting some strange results. So I simplified it to just the Alpha, with two blocks of 50% grey. My problem is that I can't seem to get a proper subtract, so where the two blocks overlap, the result should be zero/black. I've tried everything I can think of, with the Merge node, Channel Booleans, and even the Matte nodes, but nothing seems to give me a true subtract.

I'm sure I'm missing something obvious, but I just can't work out what. Could any helpful person out there offer a newb any help or advice?

Thanks in advance

al

Subtract is not what you want.
You are looking for Multiply by alpha (invert radio control on)

Many applications, use the term 'subtract' very loosely, as it resonates with users much more so than multiply by alpha. But what they are in fact doing, is exactly that.

see the difference with 2 soft edge ellipses crossing each other.