[Feature] Support for trade sanctions

[gonuke]

What is the motivation for your feature request? Please describe what you want to use it for.

The TariffRegion (#661) has support for tariffs/subsidies for trade between individual regions, but these are multipliers that make things more/less expensive, but don't explicitly prohibit trade. It would be good to also have trades be completely disallowed as if sanctions were in place. This could be another feature of the TariffRegion, or a separate region.

Describe the feature you'd like.

Enable a region to have sanctions by defining a list of regions that are sanctioned.

Describe alternatives you've considered.

Use a very large tariff.

How can this issue be closed?

Introduce capability to support sanctions in a region.

[dean-krueger]

Since (at least as far as I can remember) trades with preference < 0 (ie negative) are ignored, we could fairly easily add the ability for the adjustment of a tariff region to be set to -1. Currently if the multiplier is <= 0 the pref goes to inf, but we could instead make it go to -1 such that giving an adjustment < -1 (multiplier = 1 + adjustment) makes the bid pref go negative and become sanctions? I think that should work as a quick and dirty solution, though it may be "better" to enable an explicit one somehow in case this ignoring negative behavior changes in the future.

[gonuke]

Since (at least as far as I can remember) trades with preference < 0 (ie negative) are ignored, we could fairly easily add the ability for the adjustment of a tariff region to be set to -1. Currently if the multiplier is <= 0 the pref goes to inf, but we could instead make it go to -1 such that giving an adjustment < -1 (multiplier = 1 + adjustment) makes the bid pref go negative and become sanctions? I think that should work as a quick and dirty solution, though it may be "better" to enable an explicit one somehow in case this ignoring negative behavior changes in the future.

So, I thought your multipliers in the TariffRegion are multipliers on cost, so that a number less than 0 constitutes a subsidy and a number greater than zero constitutes a tariff. We could model a sanction as an infinite (or very large) tariff, but I worry that there may be some circumstances in which the sanctioned material may be the only material and then still flow in the DRE.

The only way to get a negative preference from the current paradigm, would be to have a negative cost, as you suggest above: an adjustment < -1. However, this means that any adjustment approaching -1 is a subsidy (benefit) and suddenly at -1, it becomes a sanction. This seems a little awkward for users...

[dean-krueger]

I agree, I was thinking about this and it would probably make the most sense to add a specific sanction feature, but then I started going down the "region features as toolkit injectable plug and play modules" road and I wasn't sure how I felt about all that (it would be cool, but I'm not totally sure how I feel about putting everything in the header for more complex features)

[dean-krueger]

Thinking about this again in the context of the new Welfare Economics plan and I feel like there may need to be changes to the way tariff regions adjust preferences (currently set up for a "flip the DRE model" of things where we may actually want it to act differently), and we may have to think about this entirely differently since I think in the old model we couldn't have negative preferences by design but now we may be moving towards thinking of DRE interactions in terms of arc weights instead of preferences and then if we're doing MC-MU (or vice versa, I forget which one gives the min objective at this exact moment) we will sometimes have negative arc values which need to be allowed.