docs: def getGradient(self,j):
          """ get rotated gradient for fixed effect i """
          i = int(self.indicator['term'][j])
          r = int(self.indicator['row'][j])
          c = int(self.indicator['col'][j])
          rv = -np.kron(self.Fstar()[i][:,[r]],self.Astar()[i][[c],:])
          return rv