title: How do I use counting related instances instead of attributes in OCL
body: <p>For example say if I had a library and a precondition of issuing a book to a borrower was that it can only be issued if it is not reference only and is not already on loan to someone else, I could say:</p>
  
  <pre><code>context Copy::issue() : Boolean
      pre: (referenceCopy and onLoan) == false
  </code></pre>
  
  <p>But that would use the attribute onLoan for the book copy. Instead I want to say this with collection-methods to count related entities?</p>