Also don't miss lib.addMetaAttrs:

> lib.addMetaAttrs { platforms = drv.meta.platforms ++ [ "x86_64-darwin" ] } drv

is a bit neater than having to go all:

> drv.overrideAttrs (oldAttrs: { meta = oldAttrs.meta // { platforms = oldAttrs.meta.platforms ++ [ "x86_64-darwin" ]; }; })

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