docs: def one_version(self, index=0):
          '''
          Leaves only one version for each object.
  
          :param index: List-like index of the version.  0 == first; -1 == last
          '''
          def prep(df):
              start = sorted(df._start.tolist())[index]
              return df[df._start == start]
  
          return pd.concat([prep(df) for _, df in self.groupby(self._oid)])