  Nonspherical collapse is modelled, under the Zeldovich approximation, by
six-dimensional random walks of the initial deformation tensor field. The
collapse boundary adopted here is a slightly-modified version of that proposed
by Chiueh and Lee (2001). Not only the mass function agrees with the fitting
formula of Sheth and Tormen (1999), but the bias function and conditional mass
function constructed by this model are also found to agree reasonably well with
the simulation results of Jing (1998) and Somerville et al. (2000),
respectively. In particular, by introducing a small mass gap, we find a fitting
formula for the conditional mass function, which works well even at small time
intervals between parent and progenitor halos during the merging history.
