Other Obligations in the GPL
The GPL doesn't grant unconditional licenses. Those who copy and distribute verbatim copies of a Program are required to:
...Conspicuously and appropriately publish on each copy an appropriate copyright notice and disclaimer of warranty; keep intact all the notices that refer to this License and to the absence of any warranty; and give any other recipients of the Program a copy of this License along with the Program. (GPL section 1.)
Those who create and distribute derivative works are also required to:
a) Cause the modified files to carry prominent notices stating that you changed the files and the date of any change. (GPL section 2)
c) If the modified program normally reads commands interactively when run, you ...
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