title: Write a function that gets the average of the numbers in L2 that are larger than the largest number in L1
body: <p>Write a function <code>average-above-max</code>, which takes 2 lists, <code>L1</code> and <code>L2</code>. <code>L1</code> and <code>L2</code> are both simple lists, which do not contain nested lists. Both lists may have non-numeric elements.</p>
  
  <p>The result of the function is the average of the numbers in <code>L2</code> that are larger than the largest number in L1.</p>
  
  <p>If there is no number in <code>L1</code>, all the numbers in <code>L2</code> should be used to calculate the average.</p>
  
  <p>If there is no number in <code>L2</code>, the average is <code>0</code>.</p>
  
  <p>For example, the result of <code>(average-above-median (list 2 'a 1) (list 'b 5 3 1))</code> should be <code>4</code>.</p>
  
  <p>And this is what I have:</p>
  
  <pre><code> (define (filter l n)
     (cond
       ((null? l) empty)
       ((number? (car l)) (cons (car l) (filter (cdr l))))
       (else (filter (cdr l)))))
  </code></pre>
  
  <p>which only picks numbers out of a list.</p>