RFC: Profiling and merging for the print edition

[Warren Block]

My proposal for merging the ISBN branch and getting out of our
two-branch problem.

First, profiling works.  A demo file of the Porter's Handbook is here:
http://wonkity.com/~wblock/profiledemo/

After applying the patch, build either the online or print version of
the book with:

   make EDITION=online clean book.html
   make EDITION=print  clean book.html

The "online" version includes all sections with edition="online" or an
edition not set.

The "print" version includes all sections with edition="print" or an
edition not set.

Actually, all we need to mark are the online (non-print) sections.  By 
default, all sections would be for the print edition unless marked as 
online.

(Difficulties: the default edition should be set in the Makefile, and
make needs to recognize that changing the EDITION setting means the
files need to be rebuilt.  Left as an exercise for the reader.)

Given that we can do all or nearly all that needs to be done for the
print edition of the Handbook with profiling, I propose the following
merge plan:

     Immediately merge the ISBN branch to head

     Start using profiling in the entire Handbook

     When ready to print the book:

       branch ISBN

       in the branch, make only the changes absolutely needed for print
       that cannot be done with profiling

       print the book

       the ISBN branch becomes historical, no new changes are merged to
       it

The first merge back from ISBN will be painful.  I think, that if I and
others pitch in, we can do this merge.  And of course, it only gets
worse as time goes on and there are more conflicts between the branches.
After that, things become much easier.