[xsl] XSL Documentation: a number is true when...

Hi there,

I'm reading http://www.w3.org/TR/xpath#function-boolean for some 
workshop. Therein is said:
>>The *boolean <http://www.w3.org/TR/xpath#function-boolean>* function 
converts its argument to a boolean as follows:
   *

     a number is true if and only if it is neither positive or negative
     zero nor NaN<<

My problem may be just lack of english language, but I need some 
explanation here. What is meant by positive or negative zero? From my 
math understandings zero is neither positive nor negative. If I drop 
that and say, a number is true when the value is neither zero nor NaN, 
would this be correct?
Ciao!
Kai

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