23 def added_symbols(old, new):  argument
24     return _symbol_difference(new, old)
27 def removed_symbols(old, new):  argument
28     return _symbol_difference(old, new)
31 def changed_symbols(old, new):  argument
33     for old_sym in old:
37         if (new_sym is not None and not new_sym in old
43 def diff(old, new):  argument
44     added = added_symbols(old, new)
45     removed = removed_symbols(old, new)
46     changed = changed_symbols(old, new)