code: def describe_features(self, traj):
          
  
          feature_descs = []
          _, mapping = md.geometry.sasa.shrake_rupley(traj, mode=self.mode, get_mapping=True)
          top = traj.topology
  
          if self.mode == "residue":
              resids = np.unique(mapping)
              resseqs = [top.residue(ri).resSeq for ri in resids]
              resnames = [top.residue(ri).name for ri in resids]
              atoms_in_res = [res.atoms for res in top.residues]
              aind_tuples = []
              # For each resdiue...
              for i,x in enumerate(atoms_in_res):
                  # For each atom in the residues, append it's index
                  aind_tuples.append([atom.index for atom in x])
              zippy = itertools.product(['SASA'],['N/A'],[self.mode], zip(aind_tuples, resseqs, resids, resnames))
          else:
              resids = [top.atom(ai).residue.index for ai in mapping]
              resseqs = [top.atom(ai).residue.resSeq for ai in mapping]
              resnames = [top.atom(ai).residue.name for ai in mapping]
              zippy = itertools.product(['SASA'],['N/A'],[self.mode], zip(mapping, resseqs, resids, resnames))
  
          feature_descs.extend(dict_maker(zippy))
  
          return feature_descs
docs: Return a list of dictionaries describing the SASA features.
  
          Parameters
          ----------
          traj : mdtraj.Trajectory
              The trajectory to describe
  
          Returns
          -------
          feature_descs : list of dict
              Dictionary describing each feature with the following information
              about the atoms participating in each SASA feature
                  - resnames: names of residues
                  - atominds: atom index or atom indices in mode="residue"
                  - resseqs: residue ids (not necessarily 0-indexed)
                  - resids: unique residue ids (0-indexed)
                  - featurizer: SASA
                  - featuregroup: atom or residue