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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(self):
 """A strcutre is defined so get, if the sequence is not already there, get the sequence from the reference Always is returned on the positive strand for the MappingGeneric :param ref_dict: reference dictionary (only necessary if sequence has not been set already) :type ref_dict: dict() """

    if not self._options.ref:
      raise ValueError("ERROR: sequence is not defined and reference is undefined")
    #chr = self.range.chr
    return self.get_sequence(self._options.ref)