code: def mom_110(self, idxi, idxj, idxk):
          
          rank_ = min(
              chaospy.bertran.rank(idxi, self.dim),
              chaospy.bertran.rank(idxj, self.dim),
              chaospy.bertran.rank(idxk, self.dim)
          )
          par, axis0 = chaospy.bertran.parent(idxj, self.dim)
          gpar, _ = chaospy.bertran.parent(par, self.dim, axis0)
          idxi_child = chaospy.bertran.child(idxi, self.dim, axis0)
          oneup = chaospy.bertran.child(0, self.dim, axis0)
  
          out = self(idxi_child, par, 0)
          for k in range(gpar, idxj):
              if chaospy.bertran.rank(k, self.dim) >= rank_:
                  out -= self.mom_111(oneup, par, k) * self.mom_111(idxi, k, 0)
          return out
docs: Backend moment for i, j, k == 1, 1, 1.