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The "Multimethods and Hierarchies" page mentions that "isa?" has special behavior when aimed at two vectors(link: 1). But the docstring of "isa?" does not mention it(link: 2).
(link: 1) http://clojure.org/multimethods
(link: 2) http://clojure.github.io/clojure/clojure.core-api.html#clojure.core/isa?