def common_string_sequence(s, t):
    """
    Find a sequence of characters that occurs, in order, in both s and t.
    The sequence need not be contiguous in either.

    Doesn't necessarily return the "best" (or longest) such sequence.
    """
    common = []
    p = 0         # position in string t
    for c in s:
        pos = t[p:].find(c)
        if pos > -1:
            common.append(c)
            p += pos + 1
    return ''.join(common)