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