***** 2014-08-21 *****

Language hierarchy + accompanying test file:
(each language extension should run all the previous langs' test files)

stlc.rkt
stlc+mod-begin.rkt
stlc+define.rkt
sysf.rkt

***** 2014-08-20 *****
- still trying to solve the issue with the impl of forall instantiation
- summary below of failures so far

#### Attempt #1: 

naive substitution during instantiation (ie redex's "subst-vars")

# Extended Description / Why I Thought It Would Work:
- instantiation is just substitution, so why not re-use the built-in
  substitution of pattern variables/templates?
- this does naive substitution, ie renames everything, even binders
- I thought it would be ok since, types are just names
  eg (forall (X) (-> X (forall (X) (-> X X)))) applied to Int becomes
     (-> Int (forall (Int) (-> Int Int))), which is valid

# Failed Because:
- if you apply a forall to a non-identifier type, ie (-> Int Int),
  the result is an invalid type

##### Attempt #2: 

Define forall as a macro that when it expands, renames binders and refs to 
something unique (ala gensym).

# Extended Description:
- requires manual expansion since I took over the expansion of the program
  via local-expand

# Failed Because:
- every type must get expanded before use, which means that every type
  constructor must get redefined to expand its subtypes
- not sure how to do this for user-defined types

# Other Notes:
- I think this means I can't expect to do any preprocessing

##### Attempt #3:

Just manually substitute, checking for the binding forms (ie forall) to avoid
capture.

Working so far.