  Calabi-Yau threefolds with infinitely many flops to isomorphic manifolds have
an extended Kahler cone made up from an infinite number of individual Kahler
cones. These cones are related by reflection symmetries across flop walls. We
study the implications of this cone structure for mirror symmetry, by
considering the instanton part of the prepotential in Calabi-Yau threefolds. We
show that such isomorphic flops across facets of the Kahler cone boundary give
rise to symmetry groups isomorphic to Coxeter groups. In the dual Mori cone,
non-flopping curve classes that are identified under these groups have the same
Gopakumar-Vafa invariants. This leads to instanton prepotentials invariant
under Coxeter groups, which we make manifest by introducing appropriate
invariant functions. For some cases, these functions can be expressed in terms
of theta functions whose appearance can be linked to an elliptic fibration
structure of the Calabi-Yau manifold.
