Fontspec SizeFeatures and recursive font definition

Question

I suspect this isn't valid code, but the docs are ambiguous.

fontspec is loaded with \fontspec[<font features>]{fontname}. BoldFont is included in "font features". Then I should be able to recursively define:

\setmainfont[SizeFeatures={
{Size={-8},BoldFont=Arial},
{Size=8-}}]{Hoefler Text}

But this gives

(fontspec) The following font options are not recognised:
(fontspec)     BoldFont=Arial

Not allowed or there's a way around it? My hopes were to make the whole font definitions in this way (with BoldFeatures={SizeFeatures=...} again and so on).

A minimal working file:

\documentclass{article}
\usepackage{fontspec}

\setmainfont[SizeFeatures={
{Size={-8},BoldFont=Arial},
{Size=8-}}]{Hoefler Text}

\begin{document}
Normal text\
{\small \textbf{Small bold}}\\
\end{document}

Edit: Using Gentoo/Linux, Texlive-2010

Answer 1

define first a bold font:

\documentclass{article}
\usepackage{fontspec}

\setmainfont[BoldFont=Arial Bold,
  SizeFeatures={
  {Size={-8},BoldFont=Arial Bold},
  {Size=8-}}]{Hoefler Text}

\begin{document}
Normal text\\

{\small \textbf{Small bold}}\\
\end{document}