As discussed on IRC, this would return a Transform-object (a to-be-created TransformBase, to be exact) without needing to copy the actual matrix. This would be equivalent to the .matrix() and .array() views already supported by ArrayBase/MatrixBase. Bug 192 could be solved simultaneously, since Map<Transform<...> >(ptr) would be equivalent to Map<Matrix<...> >(ptr).asAffine().
We could also add an .asQuaternion() method.
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