The goal of this problem set is to compile Fish abstract syntax down
to MIPS assembly code.  

You are to complete the file compile.sml at the places marked with 
IMPLEMENT_ME.  There are two big functions to right:  one collects
up the set of all variables that occur in a program, and the other
compiles Fish programs down to Mips assembly.  You do not need to
do any optimization, or have a particularly fast compiler to realize 
full credit for this assignment.  On the other hand, if you want
to try to do a few transformations, optimizations, or simplifications,
please feel free.  Just make sure to get the basic code working
correctly first.  

I've provided boiler plate code for printing out Mips code so that you
can dump the results into a file, assemble it, and load it using SPIM.

I've included a sample lexer and parser that demonstrates how PS2
could have been solved.  You might want to use this, but you are free
to use your own lexer and parser from PS2 if you prefer.  I've also
introduced the mips.ml module which contains the definition of the
mips instructions and a way to pretty-print them into assembly.

The directory tests/ is intended to hold test files that you should
construct to exercise your compiler.  I encourage you to share your
tests with other groups. 

Running make in the current directory generates an executable ps2,
which expects a file to compile, and prints out the compiled code
to standard out.

ps2 [file-to-compile]