old_contents: from .snapshot import BaseSnapshot, SnapshotFactory, SnapshotDescriptor
  from .trajectory import Trajectory
  
  from .topology import Topology
  
  from . import features
  
  from .dynamics_engine import (
      DynamicsEngine, NoEngine, EngineError,
      EngineNaNError, EngineMaxLengthError)
  
  from external_engine import ExternalEngine
  
new_contents: from .snapshot import BaseSnapshot, SnapshotFactory, SnapshotDescriptor
  from .trajectory import Trajectory
  
  from .topology import Topology
  
  from . import features
  
  from .dynamics_engine import (
      DynamicsEngine, NoEngine, EngineError,
      EngineNaNError, EngineMaxLengthError)
  
  from .external_engine import ExternalEngine
  
subject: Fix import for Py3