MODULE main
VAR
  turn : 1..2;
  p : process user(turn, 1, 2);
  q : process user(turn, 2, 1);
ASSIGN
  init(turn) := 1;
SPEC
  AG (p.state = p1 -> EF p.state = p2 )

MODULE user(turnloc, myproc, otherproc)
VAR
  state : {p1,p2,p3,p4};
ASSIGN
  init(state) := p1;
  next(state) :=
    case
      state = p1 : {p1, p2};
      state = p2 & turnloc = myproc : p3;
      state = p3 : p4;
      state = p4 : p1;
      TRUE : state;
    esac;
  next(turnloc) := 
    case
      state = p4 : otherproc;
      TRUE : turnloc;
    esac;
FAIRNESS
  running