How to depend on a system optionally?

I've read several times that it's a head ache to configure optional
dependencies with ASDF. 

How true is that? Could we perhaps provide another clause
:OPTIONALLY-DEPEND-ON in DEFSYSTEM which would load a system only if
available?

Reason is that I'm going to revive

  http://common-lisp.net/project/hyperdoc/

which is a good example of a natural optional dependency. And I'd like
to make its inclusion as easy as possible so it'll see widespread
adoption.

Thanks!,

  -T.


------------------------------------------------------------------------------
Crystal Reports - New Free Runtime and 30 Day Trial
Check out the new simplified licensing option that enables 
unlimited royalty-free distribution of the report engine 
for externally facing server and web deployment. 
http://p.sf.net/sfu/businessobjects