Minor modifications, adding comments

lib/miniImp/replacePowerMod.ml

@@ -1,10 +1,8 @@
 let rewrite_instructions (prg: Types.p_exp) : Types.p_exp =
   (* function takes a program and replaces all occurrences of powermod with
      simpler instructions *)
-  let i, o, prg = (
-    match prg with
-    | Main (i, o, exp) -> i, o, exp
-  ) in
+
+  let Main (i, o, prg) = prg in
 
   let rec contains_rewrite (prg: Types.c_exp) : Types.a_exp option =
     (* if the ast contains powermod anywhere returns the powermod, otherwise