defmulti

Creates a new multimethod with the associated dispatch function.
The docstring and attr-map are optional.
 Options are key-value pairs and may be one of:
 :default
 The default dispatch value, defaults to :default
 :hierarchy
 The value used for hierarchical dispatch (e.g. ::square is-a ::shape)
 Hierarchies are type-like relationships that do not depend upon type
inheritance. By default Clojure's multimethods dispatch off of a
global hierarchy map.  However, a hierarchy relationship can be
created with the derive function used to augment the root ancestor
created with make-hierarchy.
 Multimethods expect the value of the hierarchy option to be supplied as
a reference type e.g. a var (i.e. via the Var-quote dispatch macro #'
or the var special form).