code: public void concatInternal(String target, String [] srcs) 
      throws IOException {
      // actual move
      waitForReady();
  
      long now = FSNamesystem.now();
      unprotectedConcat(target, srcs, now);
      fsImage.getEditLog().logConcat(target, srcs, now);
    }
docs: Concat all the blocks from srcs to trg and delete the srcs files