As transformers may return lists in some places this method
        can be used to enforce a list as return value.

def visit_list(self, node, *args, **kwargs):
        """As transformers may return lists in some places this method
        can be used to enforce a list as return value.
        """
        rv = self.visit(node, *args, **kwargs)
        if not isinstance(rv, list):
            rv = [rv]
        return rv