docs: def v1_subfolder_add(request, response, kvlclient,
                       fid, sfid, cid, subid=None):
      '''Adds a subtopic to a subfolder for the current user.
  
      The route for this endpoint is:
      ``PUT /dossier/v1/folder/<fid>/subfolder/<sfid>/<cid>/<subid>``.
  
      ``fid`` is the folder identifier, e.g., ``My_Folder``.
  
      ``sfid`` is the subfolder identifier, e.g., ``My_Subtopic``.
  
      ``cid`` and ``subid`` are the content id and subtopic id of the
      subtopic being added to the subfolder.
  
      If the subfolder does not already exist, it is created
      automatically. N.B. An empty subfolder cannot exist!
  
      If the subtopic was added successfully, ``201`` status is returned.
  
      (Temporarily, the "current user" can be set via the
      ``annotator_id`` query parameter.)
      '''
      if subid is not None:
          assert '@' not in subid
      path = [
          urllib.unquote(fid),
          urllib.unquote(sfid),
          cid + (('@' + subid) if subid is not None else ''),
      ]
      path = '/'.join(path)
      new_folders(kvlclient, request).put(path)
      response.status = 201