[emidln]

kinda. The dispatch is on a single value and isa? isn't mapped over that single value if it happens to be a collection. This means you can dispatch on multiple concrete values in a collection or the isa? hierarchies of a single thing, but you don't get to isa? everything in the collection (unless you do it yourself over some limited set of things you care about; like you said, method dispatch is over ANY function).

This means that you can do something like

    (defmulti cares-about-a-and-c 
      "multimethod that cares about the first and third args" 
      (fn [a b c] [a c]))
    
    (defmethod cares-about-a-and-c [:alpha :gamma]
      [a b c]
      (prn "got :alpha and :gamma"))
    
    (defmethod cares-about-a-and-c [1 3]
      [a b c]
      (prn "got 1 and 3"))

but the following won't really work how you want it to:

    (defmethod cares-about-a-and-c [String String]
      [a b c]
      (prn "Got two things that match (isa? String)"))

   
    (cares-about-a-and-c "foo" nil "bar") ;; doesn't call our last method

You could, however, define something based on class and not isa? via your dispatch function:

    (defmulti cares-about-class-of-a-and-c
      ""
      (fn [a b c] (mapv class [a c])))

    (defmethod cares-about-class-of-a-and-c [String String]
      [a _ c]
      (println "Got the strings: " a " and " c))