VAR

  x[2]
  y[2]

DEFINE

  root := !x[0] & a0 | x[0] & b0;
  a0 := !y[0] & n1;
  b0 := y[0] & n1;
  n1 := !x[1] & a1 | x[1] & b1;
  a1 := !y[1];
  b1 := y[1];

SPEC
  
  x = y <-> root
