Following the example set by other packages, I've traditionally used /usr/local/share/ruby/site_ruby/facter/MY_PACKAGE, but what if you have a fact that isn't written in Ruby? I'm also aware of /etc/facter/facts.d but I suspect that's considered inappropriate for a package to use and should be left alone for site admins to use. The FPG didn't seem to say anything on this.
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John Florian