a := ( (x1 & y1) | ~(x1 | y1) ) & ( (x2 & y2) | ~(x2 | y2) );

b := (x1 & y2 & x2 & y1) | (x2 & y2 & ~x1 & ~y1) | (x1 & y1 & ~x2 & y2) | 
     (~x2 & ~x1 & ~y1 & y2);

satisfy !(a=b);

show !(a=b);