public static CovariantPolicy <Cb> Create <Cb>(
            Func <Cb, object> target,
            Action <CovariantPolicyBuilder <Cb> > build)
        {
            if (build == null)
            {
                throw new ArgumentNullException(nameof(build));
            }
            var policy  = new CovariantPolicy <Cb>(target);
            var builder = new CovariantPolicyBuilder <Cb>(policy);

            build(builder);
            return(policy);
        }
 public CovariantPolicyBuilder(CovariantPolicy <Cb> policy)
     : base(policy)
 {
 }