“…Different techniques may allow one to generalise the results even further. The reason why we refrain from carrying out such generalisations in the current paper is twofold: (i) we aim to write a paper accessible to a wide audience, not necessarily limited to (microlocal) analysts, and (ii) we are motivated by applications in spectral theory (see Theorems 2.3, 2.5 and [16,11]). We will address certain generalisations elsewhere.…”