Exercise 4.17.  Draw diagrams of the environment in effect when 
evaluating the expression <e3> in the procedure in the text, comparing 
how this will be structured when definitions are interpreted 
sequentially with how it will be structured if definitions are scanned 
out as described. Why is there an extra frame in the transformed 
program? Explain why this difference in environment structure can never 
make a difference in the behavior of a correct program. Design a way to 
make the interpreter implement the ``simultaneous'' scope rule for 
internal definitions without constructing the extra frame.

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There is an extra frame when <e3> is evaluated because of the let 
expression, which creates a new environment.

In a correct program, only the set! expressions will alter the values of 
the bindings in this extra environment structure, and none of these set!  
expressions will read any of these values (since (define) expressions 
must occur first, and cannot require other (define) values for 
initialization).  All subsequent read-only accesses to these values from 
<e3> will be unaffected by the fact that they are in a different 
environment frame.

An alternative approach to implement this rule is, when executing a 
procedure, to have the interpreter scan the body for all define 
expressions and remove them.  For each define expression, assign 
'*unassigned* to the variable name.  Next, for each define expression, 
evaluate it and store the result internally (in the interpreter), but do 
not bind the variable name yet.  Finally, once each is evaluated, 
replace '*unassigned* for each of the names with the corresponding value 
that was stored.  Then run the rest of the procedure body (which has had 
the define expressions removed from it).