def hard_most_common_letter(a):
    """Takes in a string of lowercase letters and returns the most common letter (if multiple, just pick one)."""
    # too tricky?
    d = {}
    for c in a:
        d[c] = d.get(c, 0) + 1
    l = list(d.items())
    l.sort(key=lambda x: -x[1])
    return l[0][0]