  Dijet production with a rapidity gap between the jets is considered as a test
ground for the production of a heavy Higgs boson via weak-boson fusion at
hadron supercolliders. It is argued that in order to perform a detailed
analysis of dijet production with a rapidity gap we need an ${\cal O}(\al_s^4)$
calculation including the relevant collinear enhancements which give structure
to the jets. Such a calculation needs not be exact, but must include the full
leading power in $\hat{s}/\hat{t}$ of the rate of dijet production in the
high-energy limit. The QCD amplitudes must be determined to the corresponding
accuracy. Accordingly, the scattering amplitudes necessary to compute the full
leading power in $\hat{s}/\hat{t}$ of dijet production to ${\cal O}(\al_s^3)$
are analysed.
