Port types?

[ajkeller34]

In discussing #11, Michael wrote:

After some thought, I agree that Cascade is not really necessary and given that things are generally more readable without it I'm in favor of making this change. I do like being able to specify what happens to the ports ahead of time though, as this would avoid some repeated code in the code for the paper. So if you have ideas for a better way to do that, that would be cool, though it's not that important.

To accomplish such functionality, it seems like a Port type would be necessary.