I think the ARC issue is the same with any other patent ... Recently somebody pointed me to a nice site showing some examples:
http://www.base.com/software-patents/examples.html
Looking at the list briefly I can find at least five patent problems using any operating system with PostgreSQL.
From my point of view having the ARC in there is just as safe / unsafe as using "Hello World" and compile it with GCC.
I don't think it possible to sue a community anyway.
Best regards and have fun reading those examples,
Hans
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