docs: 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))