Exercise 3 solution

    def group_by_first_letter(words):
        groups = {}
        for w in words:
            groups.setdefault(w[0], []).append(w)
        return groups

    print(group_by_first_letter(["apple", "avocado", "banana", "blueberry", "cherry"]))

    mon = {"ann", "bob", "cy"}
    tue = {"bob", "dee"}
    print(sorted(mon | tue))
    print(sorted(mon & tue))
    print(sorted(mon - tue))
    print(sorted(mon ^ tue))

Output:
    {'a': ['apple', 'avocado'], 'b': ['banana', 'blueberry'], 'c': ['cherry']}
    ['ann', 'bob', 'cy', 'dee']
    ['bob']
    ['ann', 'cy']
    ['ann', 'cy', 'dee']

setdefault(key, []) returns the existing list for the key, or stores and
returns a new empty one, so each word can be appended in a single line.
sorted() is used on the sets because a set has no reliable order.