  We show the method for constructing nonspreading wave packets whose shape and
motion can be general. We analyze the time evolution of nonspreading wave
packets by decomposing the Hamiltonian into two parts. Of the two, one changes
the instantaneous state, the other does not. Through this decomposition, the
time evolution operator is shown to be effectively a spatial shifting operator.
This explains why nonspreading wave packets can be nonspreading. And we show
that the part of the Hamiltonian which changes the instantaneous state governs
the motion of the nonspreading wave packets.
