Mps master: trace and poolabs: (comment only) clarify that

PoolTrivGrey knows that a white seg is an all-white seg, and 
therefore needs no greying.

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 Change: 162946
 ServerID: perforce.ravenbrook.com

mps/code/poolabs.c

@@ -470,8 +470,12 @@ void PoolTrivGrey(Pool pool, Trace trace, Seg seg)
   AVERT(Trace, trace);
   AVERT(Seg, seg);
 
-  /* @@@@ The trivial grey method probably shouldn't exclude */
-  /* the white segments, since they might also contain grey objects. */
+  /* If we had a (partially) white seg, then other parts of the */
+  /* same seg might need to get greyed. In fact, all current pools */
+  /* only ever Whiten a whole seg, so we never need to Greyen any */
+  /* part of an already Whitened seg.  So we hereby exclude white */
+  /* segs. */
+  /* @@@@ This should not really be called 'trivial'! */
   if(!TraceSetIsMember(SegWhite(seg), trace))
     SegSetGrey(seg, TraceSetSingle(trace));
 }

mps/code/trace.c

@@ -1807,6 +1807,9 @@ void TraceStart(Trace trace, double mortality, double finishingTime)
         /* of references in the segment intersects with the */
         /* approximation to the white set. */
         if (ZoneSetInter(SegSummary(seg), trace->white) != ZoneSetEMPTY) {
+          /* Note: can a white seg get greyed as well?  At this point */
+          /* we still assume it may.  (This assumption runs out in */
+          /* PoolTrivGrey). */
           PoolGrey(SegPool(seg), trace, seg);
           if (TraceSetIsMember(SegGrey(seg), trace)) {
             trace->foundation += size;