howto suspend work

[Neal Becker]

I have reached a milestone, let's call it revA.  I started working on some 
substantial change, but then realized this is not the right approach.  I 
could just hg revert (I haven't yet committed these new changes), but I'd 
like to keep it just in case.  

I could hg ci the new changes, and then hg update -r revA.  That's fine, 
except the head would still be the aborted experiment.

Suggestions?