Highlight non-closed quote

[Ph . Ivaldi]

Hello,

I created a mode (asy-mode), derived from objc-mode and I use
two-mode-mode to switch between latex-mode and asy-mode in a file .tex.

By editing this code
8<------8<------8<------8<------8<------8<------8<------8<------8<------
\documentclass{article}
\begin{document}
I'm a quote.

\begin{asy}
draw(scale(5cm)*unitcircle);
\end{asy}

\end{document}
8<------8<------8<------8<------8<------8<------8<------8<------8<------

with the cursor between '\begin{asy}' and '\end{asy}' the buffer
rocks to asy-mode, and so, all the text after the quote is highlighted
as a string would.

It is strange to highlight a string which does not exist and here it is
particularly awkward.

how to say to 'font-lock' to color only the strings between double
quote ?
-- 
Thanks for your help,
   Philippe Ivaldi.
http://piprim.tuxfamily.org/

PS: french discuss about this problem in fr.comp.applications.emacs
without convincing solution.
http://tinyurl.com/2crelu