Fill out interpreter implementation more
Fill out a basic add function
Stop pretty printing ast, just run program
Allow null programs that just return first argument
Make the add function into an integer operator HOF
Define the rest of the simple binary integer operations
Define the swap command
Redefine the pop command to throw an error when stack is empty
Reorganize the match statements of do_int_valid

Mostly for readability.  It's neat that you can destructure so deeply,
but it's more readable to do it one level at a time.
Implement the sel command
Create the stack with of_list and pass it around
Stub out nget and exec
Implement the nget command
Refactor the return value of do_command

Now it returns a list of postfix commands which is then prepended to the
current list of commands.  All existing commands just return an empty
list, this is solely to make it possible to implement exec
Implement the exec command