Who Left, Who Joined

A book club keeps a member list each month. last_month and this_month are lists of names.

Task: write member_changes(last_month, this_month). Return a tuple of two lists:

1. the people who left — on last month's list but not this month's,
2. the people who joined — on this month's list but not last month's.

Each list should be sorted alphabetically, with each name once. Either list can be empty.