old_contents: """
  Attributes
  ----------
  engine : :class:`openpathsampling.DynamicsEngine`
      referenec to the engine used to generate the snapshot
  """
  
  variables = ['engine']
  
  
  def netcdfplus_init(store):
      store.create_variable(
          'engine',
          'obj.engines',
          description="reference to the engine for the current simulation box"
      )
  
  
  @property
  def topology(snapshot):
      return snapshot.engine.topology
new_contents: """
  Attributes
  ----------
  engine : :class:`openpathsampling.DynamicsEngine`
      referenec to the engine used to generate the snapshot
  """
  
  variables = ['engine']
  
  
  def netcdfplus_init(store):
      store.create_variable(
          'engine',
          'obj.engines',
          description="reference to the engine for the current simulation box"
      )
  
  
  @property
  def topology(snapshot):
      return snapshot.engine.topology
  
  
  @property
  def masses(snapshot):
      return snapshot.topology.masses
  
subject: add masses feature to topology feature