docs: def set_meta(self, selected_meta):
          """Sets one axis of the 2D multi-indexed dataframe
              index to the selected meta data.
  
          :param selected_meta: The list of the metadata users want to index with.
  
          """
          meta_names = list(selected_meta)
          meta_names.append('sample')
          meta_index = []
          # To set the index for existing samples in the region dataframe.
          # The index size of the region dataframe does not necessarily be equal to that of metadata df.
          warnings.warn("\n\nThis method assumes that the last level of the index is the sample_id.\n"
                        "In case of single index, the index itself should be the sample_id")
          for x in meta_names:
              meta_index.append(self.meta.ix[self.data.index.get_level_values(-1)][x].values)
          meta_index = np.asarray(meta_index)
          multi_meta_index = pd.MultiIndex.from_arrays(meta_index, names=meta_names)
          self.data.index = multi_meta_index