  We present a general analysis of the bifurcation sequences of 2:2 resonant
reversible Hamiltonian systems invariant under spatial $\Z_2\times\Z_2$
symmetry. The rich structure of these systems is investigated by a singularity
theory approach based on the construction of a universal deformation of the
detuned Birkhoff normal form. The thresholds for the bifurcations are computed
as asymptotic series also in terms of physical quantities for the original
system.
