[dlc-dev] Support for non-corresponding multi-oracle DLCs

[Nadav Kohen]

Hi all,

One of the outstanding items that needs to be resolved as part of the v0
milestone is support for multi-oracle DLCs where not all oracles involved
have exactly corresponding outcome sets [1]. Let us consider Numeric
outcome and Enumerated outcome oracles separately.

*Numeric Oracle Options*:
Differences can occur between numeric oracle outcome sets when oracles have
non-equal num_digits and/or non-equal bases.

   1. Use the smallest supported outcome set (intersection of all oracles'
   outcome sets).
      - Build CETs as usual but with extra prefixes where needed (such as
      leading zeros in the case that an oracle's num_digits is greater than the
      smallest num_digits).
      - Then separately build a set of CETs to cover all outcomes where the
      non-minimal oracles have more extreme outcomes than the minimal oracles.
   2. DLC participants specify contract bounds and accept a weakened
   security model in edge cases.
      - Build CETs as usual but using max/min value for oracles if we
      require something from them that is out-of-bounds.
      - Client software should clearly communicate what the loss in
      security is to the client when bounds exceed some of the oracle's support
      (noting that you are still better than the situation where you
just ignore
      minimal oracles altogether because you validate that they are signing
      max/min value).
      - A benefit of this scheme is it still allows users to choose options
      1 or 3 if they prefer.
   3. Use the largest possible supported outcome set (union of all oracles'
   outcome sets).
      - Constructed in the same way as option 2 if the largest possible
      bounds are chosen.

I think Option 2 is superior to Option 3 as they both require the same
amount of code to implement, but an argument could be made that Option 1 is
a little simpler to implement than Option 2 while Option 2 is strictly more
expressive than Option 1.

*Enumerated Oracle Options*:
Differences can occur between enum oracle outcome sets when oracles have
non-equal enumerations.

   1. Construct a meta-enumeration
      - Map each element of the meta-enumeration to all corresponding
      oracle-enum outcomes and then build a DLC around the meta-enumeration.
   2. Directly construct a list of all allowed/agreeing outcomes
   corresponding to the future set of CETs

I can't really think of many pros/cons/differences between these two
approaches other than that the former seems to be more portable than the
second.

I'd be really interested in what people think will be the best approach for
both of these cases before I start adding handling for these cases to the
specification and the code!

[1]
https://github.com/discreetlogcontracts/dlcspecs/blob/master/v0Milestone.md#multi-oracle-support-for-non-corresponding-outcome-sets

Best,
Nadav
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