user=> (defprotocol P (plength [this]) (pget [this index]))
P
user=> (deftype Foo [v] P (plength [this] (count v)) (pget [this index] (nth v index)))
user.Foo
user=> (defn obj-seq [obj index] (lazy-seq (if-not (= index (plength obj)) (cons (pget obj index) (obj-seq obj (inc index))) nil)))
#'user/obj-seq
user=> (obj-seq (->Foo [1 2 3]))
ArityException Wrong number of args (1) passed to: user/obj-seq  clojure.lang.AFn.throwArity (AFn.java:429)
user=> (obj-seq (->Foo [1 2 3]) 0)
(1 2 3)
user=> (let [[a b c] (obj-seq (->Foo [1 2 3]) 0)] (println a) (println b) (println c))
1
2
3
nil