  The Hamiltonian flow of a classical, time-independent, conservative system is
incompressible, it is Liouvillian. The analog of Hamilton's equations of motion
for a quantum-mechanical system is the quantum-Liouville equation. It is shown
that its associated quantum flow in phase space, Wigner flow, is not
incompressible. It gives rise to a quantum analog of classical Hamiltonian
vector fields: the Wigner phase space velocity field~$\bm w$, the divergence of
which can be unbounded. The loci of such unbounded divergence form lines in
phase space which coincide with the lines of zero of the Wigner function. Along
these lines exist characteristic pinch points which coincide with stagnation
points of the Wigner flow.
