[Sab] error after upgrading from 0.51 to 0.52

[Sam Brauer]

After upgrading from 0.51 to 0.52 I found that some of my stylesheets no
longer worked.  The problem appears is related to the evaluation
of arithmetic expressions.

Here's the error message I get:
expr.cpp:1435: const class Str & Expression::tostringRef() const:
Assertion `(functor == EXF_ATOM) && (type == EX_STRING)' failed.

Here's a very simplified example that causes the error:
XML input document ------------------------------------------------
<?xml version='1.0' encoding='UTF-8'?>
<page>
<item>23</item>
</page>


XSL stylesheet  ------------------------------------------------
<?xml version='1.0' encoding='UTF-8'?>
<xsl:stylesheet xmlns:xsl='http://www.w3.org/1999/XSL/Transform'
version='1.0'>

<xsl:template match='page'>
    <page>
    item plus one = <xsl:value-of select='item + 1'/>
    </page>
</xsl:template>

</xsl:stylesheet>


Thanks,
Sam Brauer