+-- For each module (or SCC of modules), we take:
+--
+-- - the old in-core linkable, if available
+-- - an on-disk linkable, if available
+--
+-- and we take the youngest of these, provided it is younger than the
+-- source file. We ignore the on-disk linkables unless all of the
+-- dependents of this SCC also have on-disk linkables.
+--
+-- If a module has a valid linkable, then it may be STABLE (see below),
+-- and it is classified as SOURCE UNCHANGED for the purposes of calling
+-- compile.
+--
+-- ToDo: this pass could be merged with the preUpsweep.
+