  Finally, it may be noted that it is possible to conceive of untamed Maxwell's demons, that produce unconstrained violations of the second law, by simply modifying the microscopic laws in order to make it so. Zhang and Zhang (1992) provided an example of this replacing the partition in the centre of Maxwell's box with a velocity dependant potential capable of creating a pressure difference in the gas. This potential is non-Hamiltonian, so Liouville's Theorem does not hold. The phase volume of the macrostate of the gas can then be compressed and both the Boltzmann and Gibbs entropies can be systematically reduced even in macroscopically deterministic processes. Such examples, while not necessarily being presented as candidates for demons that can be constructed in the real world, are nevertheless useful in clarifying what attendant assumptions are being made when exorcisms are proposed.