added explanation for the use of reflect

There should be a better way than the current, but I have no idea how
to use reflect yet. This is the first working solution I  came up with
after reading the documentation for max 10 minutes.
implementation of Size()

I implemented Size() because it abstracts from the use of len(). If I
change the type of Set I might no longer be able to use len().
better Subset() and added Superset()