  We investigate the topological theory obtained by twisting the N=(2,2)
supersymmetric nonlinear sigma model with target a bihermitian space with
torsion. For the special case in which the two complex structures commute, we
show that the action is a Q-exact term plus a quasi-topological term. The
quasi-topological term is locally given by a closed two-form which corresponds
to a flat gerbe-connection and generalises the usual topological term of the
A-model. Exponentiating it gives a Wilson surface, which can be regarded as a
generalization of a Wilson line. This makes the quantum theory globally
well-defined.
