<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:

def add(self, name, func):
        ''' Attach a callback to a hook. '''
        if name not in self.hooks:
            raise ValueError("Unknown hook name %s" % name)
        was_empty = self._empty()
        self.hooks[name].append(func)
        if self.app and was_empty and not self._empty(): self.app.reset()