> If you train a model that f(x) = 5 and f(x+1) = 6, the number 7 doesn't have to exist in the training data for the model to give you a correct answer for f(x+2)

This is an interesting claim to me. Are there any models that exist that have been trained with a (single digit) number omitted from the training data?

If such a model does exist, how does it represent the answer? (What symbol does it use for the '7'?)