def _SeparateObsoleteHistogram(histogram_nodes):
  """Separates a NodeList of histograms into obsolete and non-obsolete.

  Args:
    histogram_nodes: A NodeList object containing histogram nodes.

  Returns:
    obsolete_nodes: A list of obsolete nodes.
    non_obsolete_nodes: A list of non-obsolete nodes.
  """
  obsolete_nodes = []
  non_obsolete_nodes = []
  for histogram in histogram_nodes:
    obsolete_tag_nodelist = histogram.getElementsByTagName('obsolete')
    if len(obsolete_tag_nodelist) > 0:
      obsolete_nodes.append(histogram)
    else:
      non_obsolete_nodes.append(histogram)
  return obsolete_nodes, non_obsolete_nodes