The right way to reuse code from another project

[Serghei]

Hello,

I would like to reuse some test helpers in bnf-mode [1] initially
created by lua-mode [2] authors.  So I'm interested in the proper
way mention authorship and copyright notice.  Both modes are
distributed under GNU GPL.  Could someone advice me the right way
to include those helpers in bnf-mode.

Thank you,

[1]: https://github.com/sergeyklay/bnf-mode
[2]: https://github.com/immerrr/lua-mode

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Serghei