title: how to make predicates function? in scheme
body: <p>i'm trying to make predicates functions </p>
  
  <p>datatype definition</p>
  
  <pre><code>(define-datatype expression expression?
    (const-exp (num number?))
    (var-exp (var symbol?))
    (zero?-exp (exp1 expression?))
    (diff-exp (exp1 expression?)
              (exp2 expression?)))
  </code></pre>
  
  <p>if argument is #2(struct:const-exp 7) then print #t</p>
  
  <p>if argument is #2(struct:var-exp x) then print #f</p>
  
  <pre><code>(define const-exp? ... ) 
  </code></pre>
  
  <p>it would be honor if you help me</p>