Programming Assignment #1
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Due 2/8 at 11:59pm

In Haskell:

1) Extend the implicitly-typed definitional interpreter for STLC 
   that we discussed in class to support sums and products. This 
   should be pretty easy.

2) Define a HOAS-style implicitly typed interpreter for STLC. 
   Define a type-preserving translation from HOAS-style STLC 
   to De Bruijn STLC.

3) Extend your De Bruijn STLC from 1) to support existentials.

4EC) Extend your De Bruijn STLC from 1) or 3) to support references.