old_contents: """Node implementation for the Toron project."""
  
  from itertools import chain
  from itertools import combinations
  
  from ._dal import dal_class
  
  
  class Node(object):
      def __init__(self, path, mode='rwc'):
          self._dal = dal_class(path, mode)
  
      @property
      def path(self):
          return self._dal.path
  
      @property
      def mode(self):
          return self._dal.mode
  
      def add_columns(self, columns):
          self._dal.add_columns(columns)
  
      def add_elements(self, iterable, columns=None):
          self._dal.add_elements(iterable, columns)
  
      def add_weights(self, iterable, columns=None, *, name, type_info, description=None):
          self._dal.add_weights(iterable, columns,
                                name=name,
                                type_info=type_info,
                                description=description)
  
      def rename_columns(self, mapper):
          self._dal.rename_columns(mapper)
  
      @staticmethod
      def _make_structure(discrete_categories):
          """Returns a category structure generated from a base of
          discrete categories::
  
              >>> node._make_structure([{'A'}, {'B'}, {'A', 'C'}])
              [set(), {'A'}, {'B'}, {'A', 'B'}, {'A', 'C'}, {'A', 'B', 'C'}]
  
          The generated structure is almost always a topology but that
          is not necessarily the case. There are valid collections of
          discrete categories that do not result in a valid topology::
  
              >>> node._make_structure([{'A', 'B'}, {'A', 'C'}])
              [set(), {'A', 'B'}, {'A', 'C'}, {'A', 'B', 'C'}]
  
          The above result is not a valid topology because it does not
          contain the intersection of {'A', 'B'} and {'A', 'C'}--the set
          {'A'}.
          """
          structure = []
          lengths = range(len(discrete_categories) + 1)
          for r in lengths:
              for c in combinations(discrete_categories, r):
                  unioned = set().union(*c)
                  if unioned not in structure:
                      structure.append(unioned)
          return structure
  
  
new_contents: """Node implementation for the Toron project."""
  
  from itertools import chain
  from itertools import combinations
  
  from ._dal import dal_class
  
  
  class Node(object):
      def __init__(self, path, mode='rwc'):
          self._dal = dal_class(path, mode)
  
      @property
      def path(self):
          return self._dal.path
  
      @property
      def mode(self):
          return self._dal.mode
  
      def add_columns(self, columns):
          self._dal.add_columns(columns)
  
      def add_elements(self, iterable, columns=None):
          self._dal.add_elements(iterable, columns)
  
      def add_weights(self, iterable, columns=None, *, name, type_info, description=None):
          self._dal.add_weights(iterable, columns,
                                name=name,
                                type_info=type_info,
                                description=description)
  
      def rename_columns(self, mapper):
          self._dal.rename_columns(mapper)
  
      @staticmethod
      def _make_structure(discrete_categories):
          """Returns a category structure generated from a base of
          discrete categories::
  
              >>> node._make_structure([{'A'}, {'B'}, {'A', 'C'}])
              [set(), {'A'}, {'B'}, {'A', 'C'}, {'A', 'B'}, {'A', 'B', 'C'}]
  
          The generated structure is almost always a topology but that
          is not necessarily the case. There are valid collections of
          discrete categories that do not result in a valid topology::
  
              >>> node._make_structure([{'A', 'B'}, {'A', 'C'}])
              [set(), {'A', 'B'}, {'A', 'C'}, {'A', 'B', 'C'}]
  
          The above result is not a valid topology because it does not
          contain the intersection of {'A', 'B'} and {'A', 'C'}--the set
          {'A'}.
          """
          structure = []  # Use list to preserve lexical order of input.
          lengths = range(len(discrete_categories) + 1)
          for r in lengths:
              for c in combinations(discrete_categories, r):
                  unioned = set().union(*c)
                  if unioned not in structure:
                      structure.append(unioned)
          return structure
  
  
subject: Correct docstring and add comment regarding 'stable order'.