code: public void concat(Path trg, Path [] psrcs, boolean restricted) throws IOException {
      String [] srcs = new String [psrcs.length];
      for(int i=0; i<psrcs.length; i++) {
        srcs[i] = getPathName(psrcs[i]);
      }
      dfs.concat(getPathName(trg), srcs, restricted);
    }
docs: THIS IS DFS only operations, it is not part of FileSystem
  move blocks from srcs to trg
  and delete srcs afterwards
  @param trg existing file to append to
  @param psrcs list of files (same block size, same replication)
  @param restricted - should the equal block sizes be enforced
  @throws IOException