Enable one derivative asset to be backed by multiple underlying assets.

[al3mart]

Motivation

It would be nice to have the possibility of managing one asset on a parachain that is backed by a subset of all the underlying assets that have been minted in a reserve as it could be Statemine/mint.

Applying this to trappist config, tUSD could be backed by a group of assets that have been agreed upon by the governance of the parachain.

This is pretty much inspired by how DAI works, where "vaults"(CDPs) are created to collateralise the minting of DAI and only CDPs can be created from a whitelist of tokens that are decided by governance.

Unclear points

I guess this involves a set of inputs from oracles to decide which quantity of tUSD would need to be created for each unit of underlying asset deposited.

Another point I am not that clear on is whether these vaults for collateral should be created on Statemine or in Trappist itself. Based on my understanding on the whole reserve system, I would say that they should exist on Trappist, leaving the schema as follows:

• A set of reserve assets exist on Statemine, Trappist governance decides on a subset of them, from their derivatives on Trappist some vaults would be created, on Trappist as well, and following a certain ratio for each vault a certain amount of tUSD would be minted on Trappist.
  Though, I would appreciate some thoughts on the above.