  It is shown that not all linear electromagnetic constitutive laws will define
almost-complex structure on the bundle of 2-forms on the spacetime manifold
when composed with the Poincare duality isomorphism, but only a restricted
class of them that includes linear spatially isotropic and some bi-isotropic
laws. Although this does not trivialize the formulation of the basic equations
equations of pre-metric electromagnetism, it does affect their reduction to
metric electromagnetism by its effect on the types of media that are reducible,
and possibly its effect on the way that such media support the propagation of
electromagnetic waves.
