  We discuss a simple example of an F-theory compactification on a Calabi-Yau
fourfold where background fluxes, together with nonperturbative effects from
Euclidean D3 instantons and gauge dynamics on D7 branes, allow us to fix all
closed and open string moduli. We explicitly check that the known higher order
corrections to the potential, which we neglect in our leading approximation,
only shift the results by a small amount. In our exploration of the model, we
encounter interesting new phenomena, including examples of transitions where D7
branes absorb O3 planes, while changing topology to preserve the net D3 charge.
