private function buildPolicy($method, callable $fn)
{
if (!method_exists('\\Shmock\\Policy', $method)) {
throw new \InvalidArgumentException("{$method} does not exist on policies");
}
$builder = new ClassBuilder();
$builder->setExtends("\\Shmock\\Policy");
$builder->addMethod($method, function (Invocation $i) use($fn) {
return $i->callWith($fn);
}, ['$className', '$methodName', '$value', '$static']);
$clazz = $builder->create();
return new $clazz();
}