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References to Non-Free Software and Documentation

A GNU program should not recommend use of any non-free program.  We
can't stop some people from writing proprietary programs, or stop other
people from using them, but we can and should avoid helping to
advertise them to new potential customers.  Proprietary software is a
social and ethical problem, and the point of GNU is to solve that
problem.
   When a non-free program or system is well known, you can mention it
in passing--that is harmless, since users who might want to use it
probably already know about it.  For instance, it is fine to explain
how to build your package on top of some non-free operating system, or
how to use it together with some widely used non-free program.
   However, you should give only the necessary information to help those
who already use the non-free program to use your program with it--don't
give, or refer to, any further information about the proprietary
program, and don't imply that the proprietary program enhances your
program, or that its existence is in any way a good thing.  The goal
should be that people already using the proprietary program will get
the advice they need about how to use your free program, while people
who don't already use the proprietary program will not see anything to
lead them to take an interest in it.
   If a non-free program or system is obscure in your program's domain,
your program should not mention or support it at all, since doing so
would tend to popularize the non-free program more than it popularizes
your program.  (You cannot hope to find many additional users among the
users of Foobar if the users of Foobar are few.)
   A GNU package should not refer the user to any non-free documentation
for free software.  Free documentation that can be included in free
operating systems is essential for completing the GNU system, so it is
a major focus of the GNU Project; to recommend use of documentation
that we are not allowed to use in GNU would undermine the efforts to
get documentation that we can include.  So GNU packages should never
recommend non-free documentation.