docs: def flatten(x):
      """
      Flatten an arbitrary depth nested list.
      """
      # Lifted from: http://stackoverflow.com/a/406822/263969
      result = []
      for el in x:
          if hasattr(el, "__iter__") and not isinstance(el, basestring):
              result.extend(flatten(el))
          else:
              result.append(el)
      return result