old_contents: def missing_openmm(*args, **kwargs):
      raise RuntimeError("Install OpenMM to use this feature")
  
  try:
      import simtk.openmm
      import simtk.openmm.app
  except ImportError:
      HAS_OPENMM = False
      Engine = missing_openmm
      empty_snapshot_from_openmm_topology = missing_openmm
      snapshot_from_pdb = missing_openmm
      snapshot_from_testsystem = missing_openmm
      to_openmm_topology = missing_openmm
      trajectory_from_mdtraj = missing_openmm
      trajectory_to_mdtraj = missing_openmm
      Snapshot = missing_openmm
      MDSnapshot = missing_openmm
  else:
      from .engine import OpenMMEngine as Engine
      from .tools import (
          empty_snapshot_from_openmm_topology,
          snapshot_from_pdb,
          snapshot_from_testsystem,
          to_openmm_topology,
          trajectory_from_mdtraj,
          trajectory_to_mdtraj
      )
  
      from . import features
  
      from .snapshot import Snapshot, MDSnapshot
  
  from openpathsampling.engines import NoEngine, SnapshotDescriptor
  
new_contents: def missing_openmm(*args, **kwargs):
      raise RuntimeError("Install OpenMM to use this feature")
  
  try:
      import simtk.openmm
      import simtk.openmm.app
  except ImportError:
      HAS_OPENMM = False
      Engine = missing_openmm
      empty_snapshot_from_openmm_topology = missing_openmm
      snapshot_from_pdb = missing_openmm
      snapshot_from_testsystem = missing_openmm
      to_openmm_topology = missing_openmm
      trajectory_from_mdtraj = missing_openmm
      trajectory_to_mdtraj = missing_openmm
      Snapshot = missing_openmm
      MDSnapshot = missing_openmm
  else:
      from .engine import OpenMMEngine as Engine
      from .tools import (
          empty_snapshot_from_openmm_topology,
          snapshot_from_pdb,
          snapshot_from_testsystem,
          to_openmm_topology,
          trajectory_from_mdtraj,
          trajectory_to_mdtraj
      )
  
      from . import features
  
      from .snapshot import Snapshot, MDSnapshot
      from . import topology
  
  from openpathsampling.engines import NoEngine, SnapshotDescriptor
  
subject: Fix backward compatiblity for MDTrajTopology