gistfile1.txt

(update-in foo [:bar :baz] ....)

What I meant was, each part of the path does an equality check of the original
compound data structure.

So, it says, give me the subsection of foo where key is *equal to* :bar.

Here, the predicate that says "key being equal to" is hard-coded.

What if it took any predicate?

That is, find 

(update-in foo [some-func some-other-func] [transform])

It first transforms foo via some-func, to get a substructure.

Then, passes that substructure to find the nested substructure.

Finally, transforms it, and puts it back in place.

Of course, in the original version, due to equality checks on keys, there can only be one such value.

In the general version taking predicates, it can match multiple.

This essentially decomplects finding and transforming.