Document CopySet/CopyTagged

[Chris-Hibbert]

What is the Problem Being Solved?

ERTP purses have a purse.getRecoverySet() method that returns a CopySet, but I couldn't find declarations or documentation for how to use them. Eventually @erights suggested getCopySetKeys(). The wonderful endojs docs do link from CopySet to CopyTagged, but neither suggests this.

Description of the Design

Please add something that will show up when searching for CopySet. I presume there are other parallel sets, maps, etc. that could also benefit from this.