Combining grammars use cases

Hi folks,

I made a PR for a first draft of a use cases document for combining
grammars. I’m sure it’s incomplete. Additional use cases eagerly
solicited.

You can see the document here:

  https://github.com/ndw/ixml/blob/comb-use-case/misc/use-cases-for-combining-grammars.md

                                        Be seeing you,
                                          norm

--
Norm Tovey-Walsh
Saxonica