The answer to this second question is less obvious, because at first it seems like it does not
really matter which variables we specify (as long as all degrees of freedom are consumed,
because the remaining variables are then uniquely determined). However, this is true only
when there is no uncertainty caused by disturbances and noise (signal uncertainty) or model
uncertainty. When there is uncertainty then it does make a difference how the solution is
implemented, that is, which variables we select to control at their setpoints.