code: def check_prior_probs(self, statements):
          
          sources = set()
          for stmt in statements:
              sources |= set([ev.source_api for ev in stmt.evidence])
          for err_type in ('rand', 'syst'):
              for source in sources:
                  if source not in self.prior_probs[err_type]:
                      msg = 'BeliefEngine missing probability parameter' + \
                          ' for source: %s' % source
                      raise Exception(msg)
docs: Throw Exception if BeliefEngine parameter is missing.
  
          Make sure the scorer has all the information needed to compute
          belief scores of each statement in the provided list, and raises an
          exception otherwise.
  
          Parameters
          ----------
          statements : list[indra.statements.Statement]
              List of statements to check