docs: def folders(self):
          """Return a map of the subfolder objects for this folder.
  
          This is a snapshot of the folder list at the time the call was made. 
          It does not update over time.
  
          The map contains MdFolder objects:
  
            maildir.folders()["Sent"]
  
          might retrieve the folder .Sent from the maildir.
          """
          entrys = self.filesystem.listdir(abspath(self._foldername()))
          regex = re.compile("\\..*")
          just_dirs = dict([(d,d) for d in entrys if regex.match(d)])
  
          folder = self._foldername()
          filesystem = self.filesystem
  
          class FolderList(object):
              def __iter__(self):
                  dirs = list(just_dirs.keys())
                  dirs.sort()
                  dirs.reverse()
                  for dn in dirs:
                      yield MdFolder(
                          dn[1:],
                          base=folder,
                          subfolder=True,
                          filesystem=filesystem
                          )
                  return
  
              def __list__(self):
                  return [dn[1:] for dn in just_dirs]
  
              def __contains__(self, name):
                  return just_dirs.__contains__(".%s" % name)
  
              def __getitem__(self, name):
                  return MdFolder(
                      just_dirs[".%s" % name][1:],
                      base=folder,
                      subfolder=True,
                      filesystem=filesystem
                      )
  
          f = FolderList()
          return f