(-x + x^3 + y^2 - x^4*y^2)/(t*x^2 - t*x^3 + t*x*y - x^2*y - 2*t*x^2*y + 
  x^3*y + 2*t*x^3*y - t*x^4*y + x^2*y^2 - x^3*y^2 - t*x*y^3 + t*x^2*y^3 - 
  t*x^3*y^3 + t*x^4*y^3)
