Drills tagged with this skill, unpassed first. Pass them all to mark the skill ready for scenarios.
merge_counts(a, b) that takes two dicts of name → count and returns a new dict where each name's count is the sum across both inputs.top_key(d) that returns the key whose value is the largest. If d is empty, return None. If there's a tie, returning any one of the tied keys is fine.letter_frequency(s) that returns a dict mapping each lowercase letter in s to how many times it appears. Treat s case-insensitively.