Channel filtering doesn't look right ...

I haven't tried it yet but I've just been reading the code

You define:

```
type Channel int
```

So using a filter that's `Channel(1) | Channel(3)` will just get me channel 3...

The C version has a macro called `Pm_Channel` which is  `1 << channel` so I think you probably want to change `Channel` to be a function that returns the same in order to generate a bitmask.
