Exercise 4.2.  Louis Reasoner plans to reorder the cond clauses in eval 
so that the clause for procedure applications appears before the clause 
for assignments. He argues that this will make the interpreter more 
efficient: Since programs usually contain more applications than 
assignments, definitions, and so on, his modified eval will usually 
check fewer clauses than the original eval before identifying the type 
of an expression.

a. What is wrong with Louis's plan? (Hint: What will Louis's evaluator 
do with the expression (define x 3)?)

b. Louis is upset that his plan didn't work. He is willing to go to any 
lengths to make his evaluator recognize procedure applications before it 
checks for most other kinds of expressions. Help him by changing the 
syntax of the evaluated language so that procedure applications start 
with call. For example, instead of (factorial 3) we will now have to 
write (call factorial 3) and instead of (+ 1 2) we will have to write 
(call + 1 2).

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a. Louis's evaluator will treat every combination as an application, 
since there is no way to distinguish between an application and any 
other list, except by eliminating the other possibilities first.

b. We must change application? and the related selectors:

(define (application? exp) (tagged-list? exp 'call))
(define (operator exp) (cadr exp))
(define (operands exp) (cddr exp))