Turns the specified flag on

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