Ok, this is part of a theme that haunts me and I would like to know if 
anyone shares my view or has a insteresting point.

The #'global-hierarchy in clojure.core is used to enable things such as 
multi-methods, and the isa? function.
Now, we can change a var with alter-var-root, and I see it mostly being 
used for configuration, as with *unchecked-math* or *warn-on-reflection*.
But for some reason the global-hierachy is private, and in a weird way 
where you can change it, as long you play the game.

Example:
(def a {:type ::a :val 1})

(defn cons-a-foo [bar in]
  (if (isa? (:type in) ::foo)
    (cons (:val in) bar)
    bar))

(cons-a-foo [] a)    ; => []
(derive ::a ::foo)   ; we change the global-hierarchy
(cons-a-foo [] a)    ; => [1]
(underive ::a ::foo) ; yet again
(cons-a-foo [] a)    ; => []

;And then isa? also supports a 3-ary version, which takes a hierarchy as:

(make-hierarchy) ; => {:parents {} :descendants {} :ancestors {}} ;omg it 
is just a map! glad I know those

This is all good, except for one reason: global-hierarchy is private.
We can't for instance try a "alternate-hierarchy" that is mostly the global 
one with some expeculations.
We lose all the other map functions in dealing with, especifically, the 
global-hierarchy.
And then we're back to oop.

If the global-hierarchy was not made private (or any function for that 
matter), then we could choose if we want to deal it with, or not.
In my experience, making something private is only worth the trouble if we 
give a complete substitute, that does everything and better.
Let's say a rocket would explode if we set it to nil.. Then, just don't.

With all that say, can we please remove the ^{:private true} from the 
global-hierarchy definition?
Sorry if I rant.

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