- Regarding point 11) of my review and the authors' answer: How do you relate the observation operator to the reduced state vector precisely? The definition of the reduced state vector is not a problem. The observations are not naturally connected to this reduced vector but potentially to a much finer one. That is why I believe the prolongation operator should come into play, explicitly or implicitly.

- Regarding point 15) of my review and the authors' answer + lines 69-72 + lines 210-213: Something still eludes me here. Does the sum of an increasing function and of a decreasing function always has a non-trivial minimum? This is what the authors seem to imply. Does the sum of x/(1+x) and 0.5/(1+x)^2 has a non-trivial minimum over the positive real numbers?