code: def addAllowedType(self, assoc_type, session_type=None):
          
          if self.allowed_types is None:
              self.allowed_types = []
  
          if session_type is None:
              available = getSessionTypes(assoc_type)
  
              if not available:
                  raise ValueError('No session available for association type %r'
                                   % (assoc_type,))
  
              for session_type in getSessionTypes(assoc_type):
                  self.addAllowedType(assoc_type, session_type)
          else:
              checkSessionType(assoc_type, session_type)
              self.allowed_types.append((assoc_type, session_type))
docs: Add an association type and session type to the allowed
          types list. The assocation/session pairs are tried in the
          order that they are added.