[('a',), ('b',), ('c',), ('d',), ('e',), ('a', 'b'), ('a', 'c'), ('a', 'd'),
    ('a', 'e'), ('b', 'c'), ('b', 'd'), ('b', 'e'), ('c', 'd'), ('c', 'e'),
    ('d', 'e')]
Listing all combinations of a list up to length n (Python)