@BOOK{refernce1,
  title={Free software, free society: Selected essays of Richard M. Stallman},
  author={Stallman, Richard},
  year={2002},
  publisher={Lulu. com},
  url={https://www.gnu.org/doc/fsfs3-hardcover.pdf},
  SN={1882114981}
}

@BOOK{refernce2,
  title={Free software, free society: Selected essays of Richard M. Stallman},
  author={Stallman, Richard},
  year={2002},
  publisher={Lulu. com},
  url={https://www.gnu.org/doc/fsfs3-hardcover.pdf},
  SN={1882114981}
}

@BOOK{refernce3,
  title={Free software, free society: Selected essays of Richard M. Stallman},
  author={Stallman, Richard},
  year={2002},
  publisher={Lulu. com},
  url={https://www.gnu.org/doc/fsfs3-hardcover.pdf},
  SN={1882114981}
}

@BOOK{refernce4,
  title={Free software, free society: Selected essays of Richard M. Stallman},
  author={Stallman, Richard},
  year={2002},
  publisher={Lulu. com},
  url={https://www.gnu.org/doc/fsfs3-hardcover.pdf},
  SN={1882114981}
}