  This manuscript has been withdrawn because of significant overlap with an
existing paper (Szabolcs Varga and Istvan Szalai, "Phase diagrams of binary
mixtures of hard rods in an external orientational field", Phys. Chem. Chem.
Phys., 2:1955-1959, 2000) of which we were, unfortunately, unaware when
submitting our manuscript.
