public override Result Eval(Env env)
{
var x = env.QueryValue(Args.First()) as Val;
if (x == null) throw new ArgumentNullException(nameof(x));
if (Args.Count() == 1)
return _evalBuiltin(x.ValName, Result.Empty);
var xs = env.QueryValue(Args.Skip(1).First()) as Val;
if (xs == null) throw new ArgumentNullException(nameof(xs));
return _evalBuiltin(x.ValName, xs.Result);
}