Letter and currency symbol being split when at end of line

Hi

I've googled this, but have got nowhere.  I'm hoping it's just an easy "use this xsl-fo tag" ...

When we specify different currencies, it's typically in the format of C$ for Canadian dollar, etc.  We're finding, if C$ is at the end of a line, it can get split between the C and the $.  Is there a way of forcing a sequence of characters to keep together, similar to how keep-together keeps lines/paragraphs together?

Thanks
Mark