docs: def add_initial_belief(self):
          """
          add initial belief tag to pomdpx model
  
          Return
          ---------------
          string containing the xml for initial belief tag
          """
          initial_belief = self.model['initial_state_belief']
          for condition in initial_belief:
              condprob = etree.SubElement(self.initial_belief, 'CondProb')
              self.add_conditions(condition, condprob)
          return self.__str__(self.initial_belief)[:-1]