This model is gauge invariant as long as only processes satisfying the following
relation are considered:

KK_sum < 6

where KK_sum is the simple scalar sum of the KK numbers of all the external 
particles. This is because we only include up to N=2 KK modes.

The general condition for gauge invariance is

KK_sum < 2(N+1).

Processes that do not satisfy this condition will be in danger of being 
calculated incorrectly because they could involve higher KK modes that are not 
included in our model.

A version of our model with N=4 is available upon request.