Added Session04 and Session05 folders

Hello,

In session05 in my folder, I posted a program for the trigram homework. I created it without looking at the solution you posted (and forgot most of the details of what we reviewed in the previous class). Please, let me know what you think if you have time to look at it.
Thank you very much!
Michel.
if you don't want to double-count, then maybe use a set, rather than a list? (I just put them all in, so that works that followed more frequently, were also more likely to be selected later..