derive

(derive tag parent) (derive h tag parent)

Records tag as a child of parent. The two-argument form rewrites the global hierarchy and answers nil; the three-argument form answers an updated hierarchy value, leaving the argument untouched -- defmulti's :hierarchy dispatches through such a value. A class spelling as tag is the keyword class answers for its values, as in isa?, so deriving a throwable class reaches every subclass.