[XeTeX] Restoring text to the default English font in Latex/fontspec/Bidi

[sjo]

Any help with the following would be much appreciated.

In a bidi (English/Hebrew) setting, how do I set matters up so that
the default English font is the standard CMR fonts (or some font say
FENG) and in the bidi/Hebrew mode the standard font is say David (or
some font FHEB). Here is a little test file that illustrates my
problem.

==========

\documentclass[12pt]{book}
\usepackage[utf8]{inputenc}
\usepackage[cm-default]{fontspec,xunicode,xltxtra}
\usepackage{bidi}
\setmainfont[Script=Hebrew]{FrankRuehl}
\newfontfamily\hfd[Script=Hebrew,Scale=0.92]{David}
\begin{document}

%%A
This text should be in CMR as cm-dfault was set.

%%B
\beginR\setRL Hebrew stuff in FrankRuehl in right to left mode

\hfd Hebrew stuff in David in right to left

%%C
\beginL\setLR I would like this text to be in CMR say, how do I do this?

==================

What I get at point C is the English test in David, not in CMR (or
whatever I would like FENG to be).

Jon