[R] question on using "warning.expression"

[JJ]

Hello:
I would like to flag certain warnings to make them
into errors.  I thought I could do this by giving
warning.expression a function to evaluate, in which I
could check the warning message and call stop if I
want.

I tried this:

options(warning.expression=expression(myfunction()))

but it does not appear to actually call myfunction. 
The function myfunction instructs that if the warning
has a certain messsage, then stop() is called.  

Can anyone help with the proper syntax for
warning.expression?  Or is there a better way to do
what I want?

Thanks,
John