Named let'' is a variant of let that has the form he and are just as in ordinary let, except that is bound within to a procedure whose body is and whose parameters are the variables in the. Thus, one can repeatedly execute the by invoking the procedure named. For example, the iterative Fibonacci procedure (section 1.2.2) can be rewritten using named let as follows:

Modify let->combination of exercise 4.6 to also support named let.
exercise 4.6 Let expressions are derived expressions, because

Implement a syntactic transformation let->combination that reduces evaluating let expressions to evaluating combinations of the type shown above, and add the appropriate clause to eval to handle let expressions