docs: def _get_function_matches(attributes_a, attributes_b, filter_set_a=None, filter_set_b=None):
          """
          :param attributes_a:    A dict of functions to their attributes
          :param attributes_b:    A dict of functions to their attributes
  
          The following parameters are optional.
  
          :param filter_set_a:    A set to limit attributes_a to the functions in this set.
          :param filter_set_b:    A set to limit attributes_b to the functions in this set.
          :returns:               A list of tuples of matching objects.
          """
          # get the attributes that are in the sets
          if filter_set_a is None:
              filtered_attributes_a = {k: v for k, v in attributes_a.items()}
          else:
              filtered_attributes_a = {k: v for k, v in attributes_a.items() if k in filter_set_a}
  
          if filter_set_b is None:
              filtered_attributes_b = {k: v for k, v in attributes_b.items()}
          else:
              filtered_attributes_b = {k: v for k, v in attributes_b.items() if k in filter_set_b}
  
          # get closest
          closest_a = _get_closest_matches(filtered_attributes_a, filtered_attributes_b)
          closest_b = _get_closest_matches(filtered_attributes_b, filtered_attributes_a)
  
          # a match (x,y) is good if x is the closest to y and y is the closest to x
          matches = []
          for a in closest_a:
              if len(closest_a[a]) == 1:
                  match = closest_a[a][0]
                  if len(closest_b[match]) == 1 and closest_b[match][0] == a:
                      matches.append((a, match))
  
          return matches