Problem with -> (solved)

gistfile1.clj

(defprotocol Graph
  (set-vertex [this v])
  (set-edge [this from to]
	    [this from to weight]))

(defrecord DirectedGraph [vs em wm]
;; A directed graph: vs is a sorted set of vertices, em is a map from vertices
;; to a sorted set of adjacent vertices, wm is a map from edges given as
;; vertex tuple [from to] to weights.
  Graph
  (set-vertex [_ v]
	      (DirectedGraph. (conj vs v)
			      (conj em [v (sorted-set)])
			      wm))
  (set-edge [this from to]
	    (set-edge this from to nil))
  (set-edge [_ from to weight]
	    (DirectedGraph. vs
			    (conj (em from) to)
			    (conj wm [[from to] weight]))))

(def test-graph (-> (DirectedGraph. (sorted-set) {} {})
		    (set-vertex :q)
		    (set-vertex :a)
		    (set-vertex :b)
		    (set-vertex :c)
		    (set-vertex :d)
		    (set-vertex :s)
		    (set-edge :q :a 8)
		    (set-edge :q :c 3)
		    (set-edge :a :b 2) ; This is line 33.
		    (set-edge :a :c 2)
		    (set-edge :a :d 1)
		    (set-edge :b :c 2)
		    (set-edge :b :s 1)
		    (set-edge :c :d 8)
		    (set-edge :d :s 7)))

;; Compiler error message:
;; error: java.lang.ClassCastException: clojure.lang.PersistentList cannot be cast to clojure.lang.IFn (graph.clj:33)