function I(x) { return x; }

function walk(obj, pre, post) {
    var k, v;
    obj = (pre||I)(obj);
    if (typeof(obj) === 'object') {
        for (k in obj) if (obj.hasOwnProperty(k)) {
            v = (post||I)(walk(obj[k], pre, post));
            if (v === undefined) {
                delete obj[k];
            } else {
                obj[k] = v;
            }
        }
    }
    return obj;
}