def METHOD_NAME(fs):
    fsa = fs
    # nuke anything already registered, just to be safe.
    while len(fs.handler_reg):
        for k in list(fs.handler_reg):
            fs.deregister_handler(k)
    # check syn-mod not in the registry
    assert 'syn-mod' not in fsa.handler_reg
    # put syn-mod in with context manager
    with fsa.handler_context({'syn-mod': SynHandlerMod}):
        # check that it is the version we expect
        assert fsa.handler_reg['syn-mod'] is SynHandlerMod
        # over-ride syn-mod with a second context manager
        with fsa.handler_context({'syn-mod': SynHandlerEcho}):
            # check that we get the second one
            assert fsa.handler_reg['syn-mod'] is SynHandlerEcho
        # and that it correctly rollsback to first value
        assert fsa.handler_reg['syn-mod'] is SynHandlerMod
    # and is empty again when we are done.
    assert 'syn-mod' not in fsa.handler_reg