  I derive unidirectional wave equations for fields propagating in materials
with both electric and magnetic dispersion and nonlinearity. The derivation
imposes no conditions on the pulse profile except that the material modulates
the propagation slowly, that is, that loss, dispersion, and nonlinearity have
only a small effect over the scale of a wavelength. It also allows a direct
term-to-term comparison of the exact bidirectional theory with its approximate
unidirectional counterpart.
