1/2 a^{a0}_{c2} b^{c0,c1}_{a1,v0} c^{c2}_{v1} eta1^{a1}_{a0} a+(v0) a+(v1) a-(c1) a-(c0)
-a^{a0}_{c2} b^{c0,c2}_{a1,v0} c^{c1}_{v1} eta1^{a1}_{a0} a+(v0) a+(v1) a-(c1) a-(c0)