Consider Weyl algebra, i.e. the algebra of $x^i$ and $p_i=\frac{\partial}{\partial x^i}$,
its elements are differential operators $F(x,p)$. Weyl algebra is $\mathbb{Z}_2$ graded, hence one ask if there exists a supertrace. It turns out that there is one
$$str\, F(x,p)=F(0,0)$$
I am looking for original references where this fact was established, some general comments are also welcome. Say, Pinczon et al in
a 2005 paper "[Supertrace and superquadratic Lie structure on the Weyl algebra, with applications to formal inverse Weyl transform][1]" mention this fact without any refs.


  [1]: http://arxiv.org/abs/math/0507092