“…Adaptive techniques based on a posteriori error estimators have become an indispensable tool and are well established for such methods; see [2,27,55] and the references therein. Error estimators have also been successfully used in the field of nonconforming discretizations [1,19,20,24] and for variational inequalities [3,13,26] such as obstacle problems; see [14,32,34,53] and for contact problems, see [11,12,18,22,31,41,61].…”