DOCBOOK: <filename>'s transition to smallcptr.char.mix, 3.x -> 4.x

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

Folks,

In the jump from DocBook 3.x to 4.x, Filename (and a few other elements)
had their content model changed from cptr.char.mix to
smallcptr.char.mix.

This means that constructs such as 

    <para>Your ssh key pair will be placed in the
      <filename><envar>$HOME</envar>/.ssh</filename> directory.</para>

which were valid in 3.x are no longer valid.

Is there a recommended alternative way to mark up this sort of text?
Wanting to mark up the environment variable, while still indicating it's
part of a filename seems fairly natural to me.

N
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