Re: dealing with extension dependencies that aren't quite 'e'

Hi.

Here's the updated patch. It fixes a couple of small problems, and
includes documentation and tests (which I placed in a new file in
src/test/modules/test_extensions, on Petr's advice).

I wanted to post this before I went on to attempt any more grammar
cleanups. Please let me know if there's anything else you'd like to
see here.

Thanks again for your review and suggestions.

-- Abhijit