title: Format verification in SPIN
body: <p>I learned Promela and Spin, but when I try verifying the model, these lines
  are returned to me.</p>
  
  <p><img src="https://i.stack.imgur.com/EIOzK.png" alt="enter image description here"></p>
  
  <p>What do they mean?</p>
  
  <p>Thanks</p>