Exercise 5.42: Using find-variable from
Exercise 5.41, rewrite compile-variable and
compile-assignment to output lexical-address instructions.  In cases
where find-variable returns not-found (that is, where the
variable is not in the compile-time environment), you should have the code
generators use the evaluator operations, as before, to search for the binding.
(The only place a variable that is not found at compile time can be is in the
global environment, which is part of the run-time environment but is not part
of the compile-time environment.332  Thus, if you wish, you may have the evaluator operations look
directly in the global environment, which can be obtained with the operation
(op get-global-environment), instead of having them search the whole
run-time environment found in env.)  Test the modified compiler on a few
simple cases, such as the nested lambda combination at the beginning of
this section.