Exercise 2.12 
   Explore an alternative operational semantics in which
   expressions that are known to be values (since the have been
   evaluated) are not evaluated again. State and prove in which way the
   new semantics is equivalent to the one given in Section 2.3. 

   Hint: It may be necessary to extend the language of expressions ore
   explicitly separate the language of values from the language of
   expressions. 

   Extend and modify the implementation of the Mini-ML interpreter to
   implement the alternative operational semantics. 

 Exercise 4.5
   Write declarations to represent natural numbers in unary and
   binary notation.

   1. Implement a translation between binary and unary representations in Elf.

   2. Formulate an appropriate representation theorem and prove it.

   3. Implement the proof of the representation theorem in Elf.

 Exercise 5.9 
   Consider the call-by-name version of Mini-ML with lazy
   constructors as sketched in Exercise 2.12. Recall that neither the
   arguments to functions, nor the arguments to constructors should be
   evaluated. 

   1. Implement an interpreter for the language and show a few
      expressions that highlight the differences in the operational
      semantics. 

   2. [ignore]

   3. Implement a suitable notion of value. 

   4. Implement the value soundness proof, i.e. call-by-name
      evaluation always returns a lazy value. 

   5. [ignore]