Exercise 4.5: Scheme allows an additional syntax
for cond clauses, (⟨test⟩ => ⟨recipient⟩).  If
⟨test⟩ evaluates to a true value, then ⟨recipient⟩ is evaluated.
Its value must be a procedure of one argument; this procedure is then invoked
on the value of the ⟨test⟩, and the result is returned as the value of
the cond expression.  For example


(cond ((assoc 'b '((a 1) (b 2))) => cadr)
      (else false))

returns 2.  Modify the handling of cond so that it supports this
extended syntax.