Clarify definition of unique parent condition

[user202729]

As in the title.

I don't think "unique parent condition" or "unique parent behavior" was defined anywhere previously, and this definition is to the best of my understanding (just come across it recently).

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[mantepse]

Looks good to me, although I'd love to have a brief look by @nbruin or @tscrim.

[nbruin]

"says": typo

[nbruin]

What I see presently on this PR are some minor corrections and modifications. They seem fine, but I don't see how the task set in the title is completed. There is still no definition of what the "unique parent condition" is. Is this ticket supposed to be ready for review?

I don't think it's the case that "Unique Parent Condition" means "Parent"+"UniqueRepresentation").
I think "Unique Parent Condition" really just means "equal means identical" (for a parent), in the way that WithEqualityByID establishes. It arises from the need in the coercion framework to determine equality of parents really quickly, so it just relies on identity in those cases. The coercion framework would be completely happy with parents that do not have any caching but where non-identical instances are not equal. It's really only the "independently reconstructible" (that from the same parameters an equal parent can be constructed) requirement that causes the caching for UniqueFactory and UniqueRepresentation.