Returns the single source name for a variant collection if it is unique,
        otherwise raises an error.

def source(self):
        """
        Returns the single source name for a variant collection if it is unique,
        otherwise raises an error.
        """
        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]