Playing with a more general implementation that allows LVars.  First baby step.
A bit more progress.  Introduce a concrete LVar type.
Incremental progress
Possibly near complete except for poll-on-put dilemma
Hmm, a small modification, but it causes a GHC panic on the self contained LVarTraceInternal.hs file.
--------------------
[1 of 1] Compiling LVarTraceInternal ( LVarTraceInternal.hs, interpreted )
ghc: panic! (the 'impossible' happened)
  (GHC version 7.6.1 for x86_64-apple-darwin):
	tc_hs_type: bang
temporary: starting down this one path (atomic counter), but now going to reverse
Wow, this particular checkin (LVarTraceInternal) causes ghci 7.6.1 to diverge
Continued sketching, see FIXME
Possible complete version plus some rearrangement
Finish things off and set the export list.  Some debugging prints that I'll now remove.
Removed debugging messages.  Also works on parfib.  But 2X slowdown vs. vanilla Trace.
significant changes: add set support, works except for callbacks
Modify LVAR rep to include callback.  Simple callback test for sets works.
First stab at IPair, putFst, putSnd.
Finish up the pairs stuff.
Fix a BUG in one of the tests (oops) and add one more test.
experimental: graph representation for BFS with LVars
Added some 'nbrs' functions.
Starting to work on graph traversal.  Doesn't typecheck yet...
LVars: Hide the implement of IVars and provide ParFuture/ParIVar instances.  Clean up tests.