  We show that Gammelgaard's formula expressing a star product with separation
of variables on a pseudo-Kaehler manifold in terms of directed graphs without
cycles is equivalent to an inversion formula for an operator on a formal Fock
space. We prove this inversion formula directly and thus offer an alternative
approach to Gammelgaard's formula which gives more insight into the question
why the directed graphs in his formula have no cycles.
