List of a

One or zero of a

A randomly chosen a

A function that takes an s and returns an a and another s i.e. s -> (s, a) aka "a stateful computation returning a) A routine that reads a file and returns an a.

In each case, the point is that you can a) write code that acts as if the k isn't there and hence b) re-use code between all of these things without changing it.

I'm going to take the opportunity to recommend http://www.cis.upenn.edu/~cis194/spring13/. Do all of the exercises, even the really basic ones. What you learn by the end will blow your mind.