docs: def nodes(self, frequency=None):
          """
          Returns all nodes of the specified frequency that are related to
          the given Session
  
          Parameters
          ----------
          frequency : str | None
              The frequency of the nodes to return
  
          Returns
          -------
          nodes : iterable[TreeNode]
              All nodes related to the Session for the specified frequency
          """
          if frequency is None:
              []
          elif frequency == 'per_session':
              return [self]
          elif frequency in ('per_visit', 'per_subject'):
              return [self.parent]
          elif frequency == 'per_study':
              return [self.parent.parent]