Return the visitor function for this node or `None` if no visitor
        exists for this node.  In that case the generic visit function is
        used instead.

def get_visitor(self, node):
        """Return the visitor function for this node or `None` if no visitor
        exists for this node.  In that case the generic visit function is
        used instead.
        """
        method = 'visit_' + node.__class__.__name__
        return getattr(self, method, None)