docs: def _update_dPrxy(self):
          """Update `dPrxy`, accounting for dependence of `Prxy` on `omega2`."""
          super(ExpCM_empirical_phi_divpressure, self)._update_dPrxy()
          if 'omega2' in self.freeparams:
              with scipy.errstate(divide='raise', under='raise', over='raise',
                              invalid='ignore'):
                  scipy.copyto(self.dPrxy['omega2'], -self.ln_piAx_piAy_beta
                          * self.Qxy * self.omega /
                          (1 - self.piAx_piAy_beta), where=CODON_NONSYN)
              scipy.copyto(self.dPrxy['omega2'], self.Qxy * self.omega,
                         where=scipy.logical_and(CODON_NONSYN, scipy.fabs(1 -
                         self.piAx_piAy_beta) < ALMOST_ZERO))
              for r in range(self.nsites):
                  self.dPrxy['omega2'][r] *= self.deltar[r]
              _fill_diagonals(self.dPrxy['omega2'], self._diag_indices)