code: def add_parent(self, parent):
          
          parent.children.add(self)
          self.parents.add(parent)
docs: Add a parent to this role,
          and add role itself to the parent's children set.
          you should override this function if neccessary.
  
          Example::
  
              logged_user = RoleMixin('logged_user')
              student = RoleMixin('student')
              student.add_parent(logged_user)
  
          :param parent: Parent role to add in.