docs: def _latex_(self):
          r"""The representation routine for transitions.
  
          >>> g1 = State("Cs", 133, 6, 0, 1/Integer(2),3)
          >>> g2 = State("Cs", 133, 6, 0, 1/Integer(2),4)
          >>> Transition(g2,g1)._latex_()
          '^{133}\\mathrm{Cs}\\ 6S_{1/2}^{4}\\ \\nrightarrow \\ ^{133}\\mathrm{Cs}\\ 6S_{1/2}^{3}'
  
          """
          if self.allowed:
              return self.e1._latex_()+'\\ \\rightarrow \\ '+self.e2._latex_()
          elif not self.allowed:
              return self.e1._latex_()+'\\ \\nrightarrow \\ '+self.e2._latex_()
          else:
              return self.e1._latex_()+'\\ \\rightarrow^? \\ '+self.e2._latex_()
  
          return self.e1._latex_()+'\\ \\nleftrightarrow \\ '+self.e2._latex_()