Exercise 4.16: In this exercise we implement the
method just described for interpreting internal definitions.  We assume that
the evaluator supports let (see Exercise 4.6).


 Change lookup-variable-value (4.1.3) to signal an error if
the value it finds is the symbol *unassigned*.

 Write a procedure scan-out-defines that takes a procedure body and
returns an equivalent one that has no internal definitions, by making the
transformation described above.

 Install scan-out-defines in the interpreter, either in
make-procedure or in procedure-body (see 4.1.3).
Which place is better?  Why?