DOCBOOK: No <co> in <userinput> ?

[Nik Clayton <nik@nothing-going-on.demon.co.uk>]

Hi,

[ Only checked this in 3.1 and 4.0, don't have 4.1 to hand yet ]

Is there any reason why <co> (callout) is allowed in <screen> but 
disallowed in <userinput>?  I'd have thought being able to mark parts
of a user's command line to refer back to in discussion would be a useful
facility.

N
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