  We show that Oeckl's boundary formalism incorporates quantum statistical
mechanics naturally, and we formulate general-covariant quantum statistical
mechanics in this language. We illustrate the formalism by showing how it
accounts for the Unruh effect. We observe that the distinction between pure and
mixed states weakens in the general covariant context, and surmise that local
gravitational processes are indivisibly statistical with no possible quantal
versus probabilistic distinction.
