Writing Program Without main Function

[Samir]

Ok.  Upon further investigation, I think I see what is going on.  
Most of the repos I am skimming are for this year's Advent of 
Code.  They structure their repo with an `app.d` file which does 
contain a `main` function but this program is structured such 
that it imports the files I was looking at earlier (which do not 
have a `main` function).

Looks like I have to get smarter on how modules, mixins and 
imports work.