The a posteriori error estimation of finite element approximations of elliptic boundary value problems has reached some state of maturity, as it is documented by a variety of monographs on this subject (cf., e.g., [1, 2, 3, 4, 5]). There are different concepts such as • residual type a posteriori error estimators, • hierarchical type a posteriori error estimators, • error estimators based on lo...