Required versus optional parts of the protocol
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   The following are part of every known implementation of the CVS
protocol (except obsolete, pre-1.5, versions of CVS) and it is
considered reasonable behavior to completely fail to work if you are
connected with an implementation which attempts to not support them.
Requests: `Root', `Valid-responses', `valid-requests', `Directory',
`Entry', `Modified', `Unchanged', `Argument', `Argumentx', `ci', `co',
`update'.  Responses: `ok', `error', `Valid-requests', `Checked-in',
`Updated', `Merged', `Removed', `M', `E'.

   A server need not implement `Repository', but in order to
interoperate with CVS 1.5 through 1.9 it must claim to implement it (in
`Valid-requests').  The client will not actually send the request.