title: How to check if the value is a number in Prolog manually?
body: <p>How to check if the given value is a number in Prolog without using built-in predicates like <code>number</code>?</p>
  
  <p>Let's say I have a list <code>[a, 1, 2, 3]</code>. I need a way to check if every element within this list is a number. The only part of the problem that bothers me is how to do the check itself without using the <code>number</code> predicate.</p>
  
  <p>The reason why I'm trying to figure this out is that I've got a college assignment where it's specifically said not to use any of the built-in predicates.</p>