functionsince v0.0-927 clojure.core/underiveEdit
(underive tag parent)
(underive h tag parent)

Source docstring:
Removes a parent/child relationship between parent and
tag. h must be a hierarchy obtained from make-hierarchy, if not
supplied defaults to, and modifies, the global hierarchy.
Source code @ clojurescript:src/main/cljs/cljs/core.cljs
(defn underive
  ([tag parent]
    (swap-global-hierarchy! underive tag parent)
  ([h tag parent]
    (let [parentMap (:parents h)
          childsParents (if (parentMap tag)
                          (disj (parentMap tag) parent) #{})
          newParents (if (not-empty childsParents)
                      (assoc parentMap tag childsParents)
                      (dissoc parentMap tag))
          deriv-seq (flatten (map #(cons (first %) (interpose (first %) (second %)))
                                  (seq newParents)))]
      (if (contains? (parentMap tag) parent)
        (reduce #(apply derive %1 %2) (make-hierarchy)
                (partition 2 deriv-seq))