str:split, str:tokenize and empty tokens

I just found out that str:split and str:tokenize extension functions 
ignore empty tokens. For example, take the following stylesheet:

<xsl:stylesheet
     version="1.0"
     xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
     xmlns:str="http://exslt.org/strings"
     extension-element-prefixes="str">
<xsl:output indent="yes"/>
<xsl:template match="/">
     <result>
         <xsl:copy-of select="str:split('a|b||c', '|')"/>
     </result>
</xsl:template>
</xsl:stylesheet>

And it will produce:

<result>
   <token>a</token>
   <token>b</token>
   <token>c</token>
</result>

This seems wrong to me, and I couldn't find anything in the EXSLT spec 
that mandates this behavior. Thoughts?

Nick