Exercise 2.54: Two lists are said to be
equal? if they contain equal elements arranged in the same order.  For
example,


(equal? '(this is a list) 
        '(this is a list))

is true, but


(equal? '(this is a list) 
        '(this (is a) list))

is false.  To be more precise, we can define equal?  recursively in
terms of the basic eq? equality of symbols by saying that a and
b are equal? if they are both symbols and the symbols are
eq?, or if they are both lists such that (car a) is equal?
to (car b) and (cdr a) is equal? to (cdr b).  Using
this idea, implement equal? as a procedure.102