code: def source(self):
          
          if len(self.sources) == 0:
              raise ValueError("No source associated with %s" % self.__class__.__name__)
          elif len(self.sources) > 1:
              raise ValueError("Multiple sources for %s" % self.__class__.__name__)
          return list(self.sources)[0]
docs: Returns the single source name for a variant collection if it is unique,
          otherwise raises an error.