Exercise:

Oski M. Wizard can't wait to implement his own Scheme evaluator (which
we'll do in week 12), so he decides to write a highly simplified one using
data-directed programming.

Oski's Scheme evaluator only handles special forms, primitive procedures,
and global variables.

    (define (simp-eval exp)
      (cond ((self-evaluating? exp) exp)
            ((variable? exp) (get exp 'bindings))
            (else
              (let ((operator (car exp))
                    (args (cdr exp)) )
                (let ((special (get operator 'special-forms)))
                  (if special
                      (apply special args)
                      (apply (simp-eval operator)
                             (map simp-eval args)) )))) ))

    (define (self-evaluating? exp)
      (or (number? exp)
          (boolean? exp)) )

    (define variable? symbol?)

To use his evaluator, Oski would type in:

> (simp-eval '(define a (+ 5 13)) )
> (simp-eval 'a)
18
> (simp-eval '(define b (* 3 6)) )
> (simp-eval 'b)
18
> (simp-eval '(if (= a b) 1 0) )
1

Help Oski finish his evaluator by adding appropriate procedures to handle
DEFINE, IF, +, *, and =.

Complete the following expressions:

(put 'define <??> <??> )
(put 'if     <??> <??> )
(put '+      <??> <??> )
(put '*      <??> <??> )
(put '=      <??> <??> )