Standardized portal_catalog index for all UID back-references

Folks,

We have the ability to index UID of items in portal_catalog.  This is
all well and good, but what index should ideally be used (or a new one
created) for tracking back-references?

Use-cases:

(1) Add-on developer wants standard way to have some kind of
referential integrity -- that is, do not delete item X if item Y
refers to it using a uuid-based reference.

...But every add-on, if they do index back-references is likely doing
so in a custom index, named specific to the problem domain of that
add-on.  This might increase proliferation of specific indexes, when a
generic, single, standardized one is appropriate.

(2) Add-on developer wants to have a portlet or view that shows what
other objects reference this object.  For example, I have an "In Use
By" object action tab with a view that lists items that "use" the
context, and that action only shows up when the item context is
actually in use.  But again, I am using an application-specific index
to implement a solution to this general problem.

(3) In a view, add-on developer wants to list some subset of
back-references, filtering on catalog brains retrieved for each
source.  Each subset need not have its own index, as filtering a
broader query of references is almost always reasonably performing.

Is this as simple as using a custom indexer adapter for
getRawRelatedItems, or is the creation of a broader-purpose keyword
index called for here?   It is possible to have a standardized index
for which add-on developers are encouraged to create indexer adapters
for?

If there is a case for a new keyword index: what should it be named?
If there is cause, I want to PLIP something for Plone 5.  If not, I'm
open to suggestions.

Sean

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