code: def _febrl_links(df):
      
  
      index = df.index.to_series()
      keys = index.str.extract(r'rec-(\d+)', expand=True)[0]
  
      index_int = numpy.arange(len(df))
  
      df_helper = pandas.DataFrame({
          'key': keys,
          'index': index_int
      })
  
      # merge the two frame and make MultiIndex.
      pairs_df = df_helper.merge(
          df_helper, on='key'
      )[['index_x', 'index_y']]
      pairs_df = pairs_df[pairs_df['index_x'] > pairs_df['index_y']]
  
      return pandas.MultiIndex(
          levels=[df.index.values, df.index.values],
          labels=[pairs_df['index_x'].values, pairs_df['index_y'].values],
          names=[None, None],
          verify_integrity=False
      )
docs: Get the links of a FEBRL dataset.