  The crease flow, replacing the Hamiltonian system used for the evolution of
crease sets on black hole horizons, is introduced and its bifurcation
properties for null hypersurfaces are discussed. We state the conditions of
nondegeneracy and typicality for the crease submanifolds, and find their normal
forms and versal unfoldings (codimension 3). The allowed boundary singularities
are thus prescribed by the Arnold-Kazaryan-Shcherbak theorem for 3-parameter
versal families, and hence identified as swallowtails and Whitney umbrellas of
particular kinds. We further present the bifurcation diagrams describing crease
evolution at the crossings of the bifurcation sets and elsewhere, and a typical
example is studied. Some remarks on the connection of these results to the
crease evolution on black hole horizons are also given.
