class_docstr: Helper class to allow construction of a list without
  having to reverse it in the end.
code: def _append(self, elem, constructor):
      if not self._tail:
          self._head = constructor(elem)
          self._tail = self._head
      else:
          self._tail.rest = constructor(elem)
          self._tail = self._tail.rest
  
      return self._head
  
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 _PListBuilder obj's underlying member method code with example usage: obj._append(elem, constructor) and returns: self