Do not include the current thread in trace lists of alternatives

Closes #121

dejafu/CHANGELOG.markdown

@@ -77,6 +77,10 @@ This project is versioned according to the [Package Versioning Policy](https://p
 
 - If bound threads are supported, the main thread when testing is bound. (#126)
 
+- Each entry in an execution trace is now in the form `(decision, alternatives, action)`.  The
+  chosen thread is no longer in the list of alternatives, which makes raw traces easier to
+  read. (#121)
+
 ### Test.DejaFu.Refinement
 
 - A blocking interference function is no longer reported as a deadlocking execution.

dejafu/Test/DejaFu/Conc/Internal.hs

@@ -146,8 +146,10 @@ runThreads sched memtype ref = go Seq.empty Nothing where
             | (fst <$> prior) `notElem` map (Just . fst) runnable' = Start chosen
             | otherwise = SwitchTo chosen
 
-          getTrc (Single a)    = Seq.singleton (decision, runnable', a)
-          getTrc (SubC   as _) = (decision, runnable', Subconcurrency) <| as
+          getTrc (Single a)    = Seq.singleton (decision, alternatives, a)
+          getTrc (SubC   as _) = (decision, alternatives, Subconcurrency) <| as
+
+          alternatives = filter (\(t, _) -> t /= chosen) runnable'
 
           getPrior (Single a)      = Just (chosen, a)
           getPrior (SubC _ finalD) = finalD