Merging doxygen modules
===

I have a large amount of code that I'm running doxygen against.  To improve performance I'm trying to break it into modules and merge the result into one set of docs.  I thought tag files would do the trick, but either I have it configured wrong or I'm misunderstanding how it works.

The directories are laid out:


    root +
         |-src+
         |    |-a
    	 |
    	 |-doc+
              |-a.dox
    	      |-main.dox
              |-main.md
              |-output+
                      |-a+
                      |  |-html
                      |-main+
                            |-html

In addition to 'a' there are other peer directories but am starting with one.

a.dox generates output and a tag file into root/doc/output

    OUTPUT_DIRECTORY=output/a
    GENERATE_TAGFILE = output/a/a.tag
    INPUT=../src/a

main.dox just inputs the markdown file that has a mainpage tag and refers to the other projects tag file.

    OUTPUT_DIRECTORY=output/main
    INPUT = main.md
    TAGFILES=output/a/a.tag=output/a/html

 1. Should this merge or link all the docs under main where I can browse 'a' globals, modules, pages, etc?  Or does this only generate links to 'a' if I explicitly cross-reference a documented entity in 'a' from inside of 'main'?
 2. If this should work, any thoughts on where my syntax is incorrect?  I've tried various ways to define TAGFILES, is the output directory relative to the main.dox file? To the a.tag file?  Or to the a/html directory?
 3. If I'm off base an TAGFILES don't work this way, is there another way to merge sets of doxygen directories into one?

Thanks.