Fix compliance to GPL v3

[cleac]

Like it is said in "How to Apply These Terms to Your New Programs":

If the program does terminal interaction, make it output a short
notice like this when it starts in an interactive mode:

<program>  Copyright (C) <year>  <name of author>
This program comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
This is free software, and you are welcome to redistribute it
under certain conditions; type `show c' for details.

The hypothetical commands show w and show c should show the appropriate
parts of the General Public License. Of course, your program's commands
might be different; for a GUI interface, you would use an "about box".

For now, application does not have any interface of such.

Source: (link)

[Marie-Joseph]

I do believe that, according to the paragraphs just before the ones you quote, Olifant is also not in compliance since it lacks the license header in each source file.

[davidak]

@Marie-Joseph header is not needed, but recommended. https://reuse.software/

Most important is a LICENSE file.