Promela and Model Description
12) LTL (Linear time Temporal Logic) and Promela
     Common languages use a logic named propositional logic, LTL extends it      with 4 temporal operators:
      1) Always ([]),  Always x >0
      2) Next, Next x >0   (What’s the operator?)
                   3) Eventually(<>), Eventually x>0
      4) Until (U), x>0 until y>0  
    
     Promela doesn't support LTL, but spin can translate it as never claim.
     ex1: x>0 until ( y >0 until z > 0)    ( suitable for specification !)
Spin will express it as  p U ( q U r) ,  p q r are the conditions