Let $a\in S(\langle \xi\rangle^2)$ be real-valued for the standard semiclassical Euclidean symbol class, with seminorms measured by the usual estimates
Assume $a(x,\xi)\ge 0$ for all $(x,\xi)\in T^*\mathbb R^n$. Then there exist constants $C>0$ and $h_0>0$, depending on finitely many of these symbol seminorms of $a$, such that