code: public void delete( Key job_key, float dummy ) { 
      if( _key != null ) {
        Log.debug(Log.Tag.Sys.LOCKS,"lock-then-delete "+_key+" by job "+job_key);
        new PriorWriteLock(job_key).invoke(_key);
      }
      Futures fs = new Futures();
      delete_impl(fs);
      if( _key != null ) DKV.remove(_key,fs); // Delete self also
      fs.blockForPending();
    }
docs: Will fail if locked by anybody other than 'job_key'