Question regarding XSLT number function

I tried to use the follow line to determine if an input was a number or not.


<xsl:if test="number( input ) = number( input )" >

I read somewhere that if input is not a number,  the function returns NaN,
and NaN is not equal to NaN so this should work. 

If my input is "75 F", Xalan 2.5.2 says it is a number, 75, but
I have tried it against xmlspy and another IDE called Stylus Studio, and
they but indicate it is not a number. 

Is there a problem with Xalan number implementation ? 

Bill Riegel
LandMark Graphics
713-839-3388