title: Count and average
body: <p>Having</p>
  
  <pre><code>(deffacts people
  (customer (name a) (money 20) )
  (customer (name b) (money 30) )
  (customer (name c) (money 40) )
  )
  </code></pre>
  
  <p>I want to calculate the average money. How can I count how many customers I have and then, the average?   </p>