MODULE main
VAR
  turn : {p,q};
  procp : process user(turn, p, q);
  procq : process user(turn, q, p);
ASSIGN
  init(turn) := p;
--SPEC

MODULE user(turn, myid, otherid)
VAR 
	state : {p1,p2};
ASSIGN
	init(state) := p1;
	next(state) := 
		case
			state = p1 & turn = myid : p2;
			state = p2 : p1;
			TRUE : state;
		esac;
	next(turn) :=
		case
			next(state) = p1 : otherid;
			TRUE : turn;
		esac;
FAIRNESS
  running