Integer square root

This essentially performs the same function as exact-integer-sqrt in math.numeric-tower.

(defn isqrt
  "Returns the greatest integer less than or equal to the principal square root
  of n."
  [n]
  {:pre [(not (neg? n))]}
  (let [n (bigint n)]
    (if (zero? n)
      n
      (loop [x (.shiftLeft BigInteger/ONE (quot (inc (.bitLength n)) 2))]
        (let [y (quot (+ x (quot n x)) 2)]
          (if (<= x y)
            x
            (recur y)))))))

I'm interested in any improvements to this code. Some specific questions:

• Should the precondition be thrown explicitly as an IllegalArgumentException?
• Is it a bad idea to shadow the parameter with a let binding?
• Should the initial guess be more explicit about the calculation it is performing by using Math.ceil and BigInteger.pow instead of inc/quot and BigInteger.shiftLeft?