In this exercise we implement the method just described for interpreting internal definitions. We assume that the evaluator supports let (see exercise 4.6)
a. Change lookup-variable-value (section 4.1.3) to signal an error if the value it finds is the symbol *unassigned*.

b. 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.
ction 4.1.3). Which place is better? Why?