docs: def get_fragment(self, list_of_indextuples, give_only_index=False,
                       use_lookup=None):
          """Get the indices of the atoms in a fragment.
  
          The list_of_indextuples contains all bondings from the
          molecule to the fragment. ``[(1,3), (2,4)]`` means for example that the
          fragment is connected over two bonds. The first bond is from atom 1 in
          the molecule to atom 3 in the fragment. The second bond is from atom
          2 in the molecule to atom 4 in the fragment.
  
          Args:
              list_of_indextuples (list):
              give_only_index (bool): If ``True`` a set of indices
                  is returned. Otherwise a new Cartesian instance.
              use_lookup (bool): Use a lookup variable for
                  :meth:`~chemcoord.Cartesian.get_bonds`. The default is
                  specified in ``settings['defaults']['use_lookup']``
  
          Returns:
              A set of indices or a new Cartesian instance.
          """
          if use_lookup is None:
              use_lookup = settings['defaults']['use_lookup']
  
          exclude = [tuple[0] for tuple in list_of_indextuples]
          index_of_atom = list_of_indextuples[0][1]
          fragment_index = self.get_coordination_sphere(
              index_of_atom, exclude=set(exclude), n_sphere=float('inf'),
              only_surface=False, give_only_index=True, use_lookup=use_lookup)
          if give_only_index:
              return fragment_index
          else:
              return self.loc[fragment_index, :]