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A. Appendix and A. , 1 General construction of R around corners In this section we generalize the tilings method presented in section 3 to any corner of angle ? ? 2?Q Recall that we define ? = 2? p p+q in ? 1 , with p, q > 0, p = q and p + q even Proceeding as in section 3, one builds admissible rotation-and symmetry-based operators R adm , and then take the average of these admissible operators to obtain the desired result (that is operators R and R with the same minimal norm as in [2]) We propose min(p, q) admissible operators below based on a simple algorithm, Consider for instance that p < q. One constructs p admissible operators