gistfile1.clj

(defn pluso [t1 t2 s]
     (fn goal [a]
       (let [args (map (partial walk a) [t1 t2 s])
             fresh? (map lvar? args)
             ground? (map not fresh?)]
         (cond
           (= [true true true]    ground?) (if (= s (+ t1 t2)) a nil)
           (= [true true false]   ground?) (unify a s  (+ t1 t2))
           (= [true false true]   ground?) (unify a t2 (- s t1))
           (= [false true true]   ground?) (unify a t1 (- s t2))
           :else a))))

gistfile2.clj

(run* [q] 
  (fresh [a] 
    (pluso q 4 a)
    (== a 7)))