old_contents: """
  This file contains the base classes for topology proposals
  """
  
  
  from collections import namedtuple
  Proposal = namedtuple('Proposal',['old_topology','new_topology','ln_selection', 'new_to_old_atom_map'])
  
  class Transformation(object):
      """
      This defines a type which, given the requisite metadata, can produce Proposals (namedtuple)
      of new topologies.
      
      Arguments
      --------
      current_topology : simtk.openmm.app.Topology object
          The topology which currently defines the system
      current_system : simtk.openmm.System object
          The system object with all relevant parameters
      metadata : object
          Data necessary for the generation of proposals
      """
      
      def __init__(self, current_topology, current_system, metadata):
          pass
      
      def propose(self):
          """
          Base interface for proposal method. Raises NotImplementedError
          
          Returns
          -------
          proposal : Proposal
              NamedTuple of type Proposal containing forward and reverse
              probabilities, as well as old and new topologies and atom
              mapping
          """
          raise NotImplementedError
  
  
new_contents: """
  This file contains the base classes for topology proposals
  """
  
  
  from collections import namedtuple
  TopologyProposal = namedtuple('Proposal',['old_topology','new_topology','ln_selection', 'new_to_old_atom_map'])
  
  class Transformation(object):
      """
      This defines a type which, given the requisite metadata, can produce Proposals (namedtuple)
      of new topologies.
      
      Arguments
      --------
      current_topology : simtk.openmm.app.Topology object
          The topology which currently defines the system
      current_system : simtk.openmm.System object
          The system object with all relevant parameters
      metadata : object
          Data necessary for the generation of proposals
      """
      
      def __init__(self, current_topology, current_system, metadata):
          pass
      
      def propose(self):
          """
          Base interface for proposal method. Raises NotImplementedError
          
          Returns
          -------
          proposal : TopologyProposal
              NamedTuple of type TopologyProposal containing forward and reverse
              probabilities, as well as old and new topologies and atom
              mapping
          """
          raise NotImplementedError
  
  
subject: clarify name of TopologyProposal