Exactly how do variants work?

[Ian Wadham]

My Macports installation has the following ports installed:
    qt4-mac @4.7.4_1+mysql+quartz
    kdegames4 @4.8.1_0

I would like to add variants +examples and +demos to qt4-mac and
variant +docs to kdegames4.  These variants do not require all of
qt4-mac and kdegames4 to be downloaded, re-built and reinstalled.
They are just extra files that can be separately installed and/or built.

What port commands should I use to add a variant to an installed port?
Do I need to uninstall and re-install the whole port?

Also, to what extent are variant names local or global?

I seem to remember that, when I added +mysql to qt4-mac, every other
port in the tree that had a +mysql variant also got built.  AFAIK +mysql
for qt4-mac just adds some optional DB access classes to the library and
builds a MySql "driver" for Qt to use.

If variant names are indeed global, how would I avoid having every installed
port that has variants +examples, +demos or +docs being re-installed?

There are quite a few of them …

Thanks, in advance.  I have gone right through "man port" and the Macports
Guide looking for answers to the above questions.

Cheers, Ian W.