1/2 a^{c0,a0}_{v0,v1} b^{a1}_{c1} c^{c1}_{a2} eta1^{a2}_{a1} a+(v0) a+(v1) a-(a0) a-(c0)
-1/2 a^{c0,a1}_{v0,v1} b^{a0}_{c1} c^{c1}_{a2} eta1^{a2}_{a1} a+(v0) a+(v1) a-(a0) a-(c0)