  An exact momentum update useful for particle codes was previously given. The
expressions involved were unwieldy. By treating the momentum as a
representation of SU(2)xSU(2) instead of SO(3,1), a more compact expression for
the exact momentum update is obtained. An expansion in powers of the timestep
can be formulated such that invariance properties are exactly preserved, and
push rates comparable to the standard Boris pusher are obtained.
