  Taking into account the recent suggestion that a short gamma-ray burst (GRB)
looks like the first 1 sec of a long GRB, we propose that the jet of a GRB
consists of multiple sub-jets or sub-shells (i.e., an inhomogeneous jet model).
The multiplicity of the sub-jets along a line of sight n_s is an important
parameter. If n_s is large (>> 1) the event looks like a long GRB, while if n_s
is small (~1) the event looks like a short GRB. If our line of sight is
off-axis to any sub-jets, the event looks like an X-ray flash or an X-ray rich
GRB. The log-normal distribution of durations of short and long GRBs are also
suggested in the same model.
