code: def _update_prx(self):
          
          qx = scipy.ones(N_CODON, dtype='float')
          for j in range(3):
              for w in range(N_NT):
                  qx[CODON_NT[j][w]] *= self.phi[w]
          frx = self.pi_codon**self.beta
          self.prx = frx * qx
          with scipy.errstate(divide='raise', under='raise', over='raise',
                  invalid='raise'):
              for r in range(self.nsites):
                  self.prx[r] /= self.prx[r].sum()
docs: Update `prx` from `phi`, `pi_codon`, and `beta`.