$E = b1*a2*a3*a4 + a1*b2*a3*a4 + a1*a2*b3*a4 + a1*a2*a3*b4 +
b1*b2*a3*a4 + b1*a2*b3*a4 + b1*a2*a3*b4 +
a1*b2*b3*a4 + a1*b2*a3*b4 + a1*a2*b3*b4 +
b1*b2*b3*a4 + b1*b2*a3*b4 + b1*a2*b3*b4 + a1*b2*b3*b4 + b1*b2*b3*b4;