docs: def get_parents(self):
          """
          Add the parents to BIF
  
          Returns
          -------
          dict: dict of type {variable: a list of parents}
  
          Example
          -------
          >>> from pgmpy.readwrite import BIFReader, BIFWriter
          >>> model = BIFReader('dog-problem.bif').get_model()
          >>> writer = BIFWriter(model)
          >>> writer.get_parents()
          {'bowel-problem': [],
           'dog-out': ['bowel-problem', 'family-out'],
           'family-out': [],
           'hear-bark': ['dog-out'],
           'light-on': ['family-out']}
          """
          cpds = self.model.get_cpds()
          cpds.sort(key=lambda x: x.variable)
          variable_parents = {}
          for cpd in cpds:
              variable_parents[cpd.variable] = []
              for parent in sorted(cpd.variables[:0:-1]):
                  variable_parents[cpd.variable].append(parent)
          return variable_parents