Exercise 3.8: When we defined the evaluation
model in 1.1.3, we said that the first step in evaluating an
expression is to evaluate its subexpressions.  But we never specified the order
in which the subexpressions should be evaluated (e.g., left to right or right
to left).  When we introduce assignment, the order in which the arguments to a
procedure are evaluated can make a difference to the result.  Define a simple
procedure f such that evaluating 


(+ (f 0) (f 1))

will return 0 if
the arguments to + are evaluated from left to right but will return 1 if
the arguments are evaluated from right to left.