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If you have the SpamAssassin source tree, well and good. If you got it from something like an RPM somewhere that did not include source, get the corresponding source or fetch Razor2.patch from http://www.spamassassin.org/released/Razor2.patch
Note: if you are using the official SpamAssassin RPMs, the Razor2.patch file is located in your documentation directory, usually /usr/share/doc/spamassassin-<version>.
See README and Razor2.patch in the source:
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