Matching two consecutive hyphens

OK, here's a plain old XSL question which seems like it ought to be easy.  And
yet.

The XML I'm given as input occasionally includes two consecutive hyphens,
which I need to test for and, if present, insert a line break.  However, two
consecutive hyphens in a standard contains() test of course throws an error:

<xsl:if test="contains(.,'--')">...

What's a good way to check for those two consecutive hyphens?

Thanks

Michele