<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def extract_users(self):
 """ extract user info """

        email_set = set()
        users_dict = {}

        self.verbose('going to extract user information from retrieved nodes...')

        for node in self.old_nodes:
            email_set.add(node.email)

            if node.email not in users_dict:
                users_dict[node.email] = {
                    'owner': node.owner
                }

        self.email_set = email_set
        self.users_dict = users_dict

        self.verbose('%d users extracted' % len(email_set))