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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 get_sequence(self,ref):
 """get a sequence given a reference"""

    strand = '+'
    if not self._options.direction:
      sys.stderr.write("WARNING: no strand information for the transcript\n")
    if self._options.direction: strand = self._options.direction
    seq = ''
    for e in [x.range for x in self.exons]:
      seq += str(ref[e.chr][e.start-1:e.end])
    if strand == '-':  seq = rc(seq)
    return Sequence(seq.upper())