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What's going on with Line 56 where obrms is added if Eigen3 is present? Can you work out whether this dependency is true?

It is - need the matrix math.

Then I'm very confused.

I have a distinct memory of trying to remove this dependency from the cmake file and it not working, but I've just tried it and it seems fine - obrms uses the built-in matrix3x3 rather than an eigen matrix so it isn't obvious to me why the current code needs the EIGEN dependency.