  We study aspects of worldsheet instantons relevant to a heterotic standard
model. The non-simply connected Calabi-Yau threefold used admits Z_3 x Z_3
Wilson lines, and a more detailed investigation shows that the homology classes
of curves are H_2(X,Z)=Z^3+Z_3+Z_3. We compute the genus-0 prepotential, this
is the first explicit calculation of the Gromov-Witten invariants of homology
classes with torsion (finite subgroups). In particular, some curve classes
contain only a single instanton. This ensures that the Beasley-Witten
cancellation of instanton contributions cannot happen on this (non-toric)
Calabi-Yau threefold.
