docs: def _set_flag(self, flag):
          """Turns the specified flag on"""
          self.folder._invalidate_cache()
          # TODO::: turn the flag off when it's already on
          def replacer(m):
              return "%s/%s.%s%s" % (
                  joinpath(self.folder.base, self.folder.folder, "cur"),
                  m.group("key"),
                  m.group("hostname"),
                  ":2,%s" % (
                      "%s%s" % (m.group("flags"), flag) if m.group("flags") \
                          else flag
                      )
                  )
          newfilename = self.msgpathre.sub(replacer, self.filename)
          self.filesystem.rename(self.filename, newfilename)
          self.filename = newfilename