<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def choose(s, possibilities, threshold=.6):
 """ Returns the closest match to string s if exceeds threshold, else returns None """

    if not possibilities: return None
    if s in possibilities: return s
    if s == '': return None
    startswith = [x for x in possibilities if x.lower().startswith(s.lower())]
    if len(startswith) == 1: return startswith[0]
    contained = [x for x in possibilities if s.lower() in x.lower()]
    if len(contained) == 1: return contained[0]
    best = max([(x, Levenshtein.jaro_winkler(s, x, .05)) for x in possibilities], key=itemgetter(1))
    if best[1] < threshold:
        #print 'did you mean %s?' % best[0]
        return None
    return best[0]