The SeenThread queue shouldn't be in the header file as it's only used by the implementation file. Only the check_seen_thread() is used outside PlaceholderTable and should be public. check_seen_thread()is essentially called once per super class loading and once per instance class loading, so doesn't require inlining.
I also added logging to the placeholder table. The entries in the PlaceholderTable are used as defacto per-class locks for class loading and for checking for ClassCircularityError.
I might remove the logging once I've written a wiki page describing how it works for bootstrap (class loader == NULL), parallelCapable and non-parallelCapable class loaders.