  Kitaev's Pfaffian formula equates the ground-state fermion parity of a closed
system to the sign of the Pfaffian of the Hamiltonian in the Majorana basis.
Using Klich's theory of full counting statistics for paired fermions we
generalize the Pfaffian formula to account for quantum fluctuations in the
fermion parity of an open subsystem. A statistical description in the framework
of random-matrix theory is used to answer the question when a vanishing fermion
parity in a superconductor fusion experiment becomes a distinctive signature of
an isolated Majorana zero-mode.
