1/4 a^{a0,a3}_{c0,c1} b^{a1,a2}_{c3,a4} c^{c3}_{c2} eta1^{a4}_{a3} a+(c0) a+(c1) a+(c2) a-(a2) a-(a1) a-(a0)
-1/2 a^{a0,a3}_{c0,c3} b^{a1,a2}_{c1,a4} c^{c3}_{c2} eta1^{a4}_{a3} a+(c0) a+(c1) a+(c2) a-(a2) a-(a1) a-(a0)