docstr: Return the length of the list, computed by traversing it.
  
  This is obviously O(n) but with the current implementation
  where a list is also a node the overhead of storing the length
  in every node would be quite significant.
code: def __len__(self):
      """
      Return the length of the list, computed by traversing it.
  
      This is obviously O(n) but with the current implementation
      where a list is also a node the overhead of storing the length
      in every node would be quite significant.
      """
      return sum(1 for _ in self)
  
imports: from collections.abc import Sequence, Hashable
  from numbers import Integral
  from functools import reduce
  from typing import Generic, TypeVar
label_desc: Use image node_id 2 for calling the _PListBase obj's underlying member method code with example usage: obj.__len__() and returns: sum