x4, x3, x2, x1, 
2-13/2*x1+x1^2*x2-1/2*x3
-x1^2*x2+6*x1+5*x2-5*x4
2-13/2*x3+x3^2*x4-1/2*x1
6*x3-x3^2*x4+1+1/2*x2-1/2*x4
