Well, let me describe what I'm trying to do, and maybe you can suggest
a better way to do it.

I am keeping 'instructions" for generating inflectional paradigms of a
language (Nahuatl) in XML.  I transform the XML files in various ways,
depending on what I'm trying to do.

The "instructions" take the form of features that I want to hold
constant for a given paradigm, and features I want to allow to vary.
So for example I might want to generate all the tenses of some set of
verbs, but holding subject person, object person, negation etc.
constant.  Another time I might want to generate all the combinations
of subject and object person forms in the present tense.

I find that I re-use lots of combinations--e.g. hold tense constant at
'present', the negation at 'positive', not use any directional affixes,
and then generate all the person combos; then hold tense constant at
'future', generate person combos, etc.  So a typical combinatin that I
want to re-use consists of the settings for tense, negation, and
directionals.  That requires three XML elements.  And I may re-use that
set of three elements in several different XML files, so I want to
collect the set into an external file where it can be defined as an
external entity, along with other commonly re-used entities.  That file
can then be loaded into various XML files.  Which entities a given XML
file actually makes use of is of course up to that XML file.

Basically, the idea strikes me as being parallel to the use of files of
library functions in a traditional programming language.  You
(statically or dynamically) link in the library file whenever your
executable needs to make use of a pre-defined function.  In my case,
what I want to do is more like static linking: I'm including in the
final XML file the pre-defined external entities.

Does that make sense?  Right now, because of the problem I was having
with the XML way of doing things, I'm just #include-ing that external
file.  Is there an XMLish way to do that, which avoids the problems
that triggered this thread?