(defn -fixc
  ([x f reifier] (-fixc x f nil reifier))
  ([x f runnable reifier]
     (reify
       clojure.lang.IFn
       (invoke [this a]
         (let [x (walk a x)]
           ((composeg (f x a reifier) (remcg this)) a)))
       IConstraintOp
       (rator [_] `fixc)
       (rands [_] (if (vector? x) x [x]))
       IReifiableConstraint
       (reifyc [c v r a]
         (if (fn? reifier)
           (reifier c x v r a)
           (let [x (walk* r x)]
             `(fixc ~x ~reifier))))
       IRunnable
       (runnable? [_ a]
         (if (fn? runnable)
           (runnable x a)
           (not (lvar? (walk a x)))))
       IConstraintWatchedStores
       (watched-stores [this] #{::subst}))))