docs: 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)