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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def sequence(db, chrom, start, end):
 """ return the sequence for a region using the UCSC DAS server. note the start is 1-based each feature will have it's own .sequence method which sends the correct start and end to this function. 'caacttag' """

    url = "http://genome.ucsc.edu/cgi-bin/das/%s" % db
    url += "/dna?segment=%s:%i,%i"
    xml = U.urlopen(url % (chrom, start, end)).read()
    return _seq_from_xml(xml)