human
Solve the following python programming problem given the following code:
def add(self, rule, module) : 
    '
        Add a Module to this Module under the given rule. All rules within the
        sub-module will be accessible to this Module, with their rules prefixed
        by the rule provided here.

        For example::

            module_one = Module()

            @module_one.route("/fish")
            def fish(request):
                return "This is fish."

            module_two = Module()

            module_two.add("/pie", module_one)

        Given that code, the request handler ``fish`` would be available from
        the Module ``module_two`` with the rules ``/pie/fish``.
        ' 
    if isinstance (module, Application) : 
        raise TypeError ('Applications cannot be added as modules.') 
module._parents.add (rule) 
    if (not ('/' in rule)) : 
        rule = ('/' + rule) 
self._modules [rule] = module 
    self._recalculate_routes () 

    
    

    Fix the buggy line: module._parents.add (rule)
assistant
#Here is Python code to fix the buggy line: module.

module._parents.add (self)