[GS] How would I code this ability?

[Pizza Guy]

I wanted to know if it was possible to code an ability that gives the a bonus to every international trade route based on every domestic trade going to the city of origin. IE: The more domestic trade routes going to a city, the larger bonus it gets from international trade routes.

How would I go about doing something like this?

edit: I also haven't figured out the exact numbers I want to use for the bonus, so it would be best if it was variable.

 

[Pizza Guy]

Update: Found an effect I think will work for what I want.

'EFFECT_ADJUST_CITY_TRADE_ROUTE_YIELD_PER_LOCAL_BONUS_RESOURCE_FOR_INTERNATIONAL'