This module may be used to implement modes for clients/servers. A
common scheme is to have two modes, Test, and Production, and to
want to maintain the invariant that clients in mode Test may not
talk to servers in mode Production, and that clients in mode
Production may not talk to servers in mode Test. Versioned
connection will check that the mode of the client is the same as
the mode of the server.
If you don't care about modes, just use Dont_care_about_mode.