function after(count, callback, err_cb) {
    var bail = false
    err_cb = err_cb || noop
    proxy.count = count

    return (count === 0) ? callback() : proxy

    function proxy(err, result) {
        if (proxy.count <= 0) {
            throw new Error('after called too many times')
        }
        --proxy.count

        // after first error, rest are passed to err_cb
        if (err) {
            bail = true
            callback(err)
            // future error callbacks will go to error handler
            callback = err_cb
        } else if (proxy.count === 0 && !bail) {
            callback(null, result)
        }
    }
}
Gets the keys for an object.

@return {Array} keys
@api private