Exercise 3.57.  How many additions are performed when we compute the nth 
Fibonacci number using the definition of fibs based on the add-streams  
procedure? Show that the number of additions would be exponentially 
greater if we had implemented (delay <exp>) simply as (lambda () <exp>), 
without using the optimization provided by the memo-proc procedure 
described in section 3.5.1.

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The zeroth and first elements are provided in the definition, and for 
n=2 or greater, there are n-1 addition operations performed between the 
time fibs in defined and the first time the nth element of the stream is 
accessed.

Without any memoization, calculating the nth element would first require 
calculating the (n-1)th and (n-2)th element separately, and then adding 
them.

Additions(n) = 0 if n = 0.
               0 if n = 1.
               Additions(n-1) + Additions(n-2) + 1 otherwise.

This is O(2ⁿ).