WIP ChainGenerator.Alternative: fixup youngestStableA defition

I /think/ this is an improvement, but I'm not sure yet if it's fully correct.

ouroboros-consensus/src/unstable-consensus-testlib/Test/Ouroboros/Consensus/ChainGenerator/Adversarial.hs

@@ -170,11 +170,24 @@ checkAdversarialChain recipe adv = do
                 -- the rest of the function won't force this since there are no
                 -- adversarial active slots
                 error "dead code"
-            kPlus1stYoungest = case BV.findIthEmptyInV S.inverted vH (C.toIndex arPrefix C.+ k) of
-              BV.JustFound kPlus1st -> C.fromWindow winH $ kPlus1st C.+ d
-              BV.NothingFound       -> C.Count d
+            kPlus1stYoungestPlusDelta = case BV.findIthEmptyInV S.inverted vH (C.toIndex arPrefix C.+ k) of
+              BV.JustFound kPlus1st -> C.fromWindow winH kPlus1st C.+ d
+              BV.NothingFound       ->
+                -- If the honest chain didn't reach @k+1@ before it ended, then
+                -- the adversary cannot acclerate before the honest chain ends.
+                --
+                -- The following value is inaccurate, since it is definitely
+                -- less than @d@ after the @k+1@st block. However, it's great
+                -- enough to prevent the test from ending while the adversary
+                -- has @k@ or more active slots remaining.
+                --
+                -- TODO if the honest chain has @x < k+1@ remaining blocks,
+                -- then @(k+1) - x + d@ is the exact answer, isn't it? By
+                -- appeal to the conservative assumption that slots after 'Len'
+                -- are active on the honest schema.
+                C.Count d -- C.windowLast winH
          in
-            max sYoungest kPlus1stYoungest
+            max sYoungest kPlus1stYoungestPlusDelta
 
     checkRaces = do
         let iterH =