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We went with @@HOMEBREW_PREFIX@@ for external patches and I feel this is the better option to avoid chances of collisions with actual HOMEBREW_PREFIX usage.

In Homebrew/core, the only formulae this impacts is:

• libxmlsec1 - https://github.com/Homebrew/homebrew-core/blob/main/Formula/lib/libxmlsec1.rb#L65
• pypy* - these are incorrect inreplaces since the code is trying to fetch environment variable, but it luckily still applies patch

The latter does mean odisabled will cause issues. Will need to move patch to formula-patches to avoid this.