code: public void registerBThread(BThreadSyncSnapshot bt) {
          recentlyRegisteredBthreads.add(bt);
          addBThreadCallback.ifPresent(cb -> cb.bthreadAdded(this, bt));
      }
docs: Registers a BThread into the program. If the program started, the BThread
  will take part in the current bstep.
  
  @param bt the BThread to be registered.