sums.xhtml

<xh:html
    xmlns:xh="http://www.w3.org/1999/xhtml"
    xmlns:xf="http://www.w3.org/2002/xforms"
    xmlns:xxf="http://orbeon.org/oxf/xml/xforms"
    xmlns:xs="http://www.w3.org/2001/XMLSchema">
    <xh:head>
        <xf:model xxf:expose-xpath-types="true">
            <xf:instance>
                <data>
                    <a/>
                    <b/>
                    <c/>
                    <sum1/>
                    <sum2/>
                </data>
            </xf:instance>
            <xf:bind ref="a | b | c | sum2" type="xs:decimal"/>
            <xf:bind ref="sum1" type="xf:decimal"/>
            <xf:bind ref="sum1" calculate="sum((../a, ../b, ../c))"/>
            <xf:bind ref="sum2" calculate="sum((../a, ../b, ../c)[string() castable as xs:decimal], 0.0)"/>
        </xf:model>
    </xh:head>
    <xh:body>
        <xf:input ref="a"/>
        <xf:input ref="b"/>
        <xf:input ref="c"/>
        <xf:output ref="sum1"><xf:label>Sum only if all values are correct:</xf:label></xf:output>
        <xf:output ref="sum2"><xf:label>Sum all correct values and ignore others:</xf:label></xf:output>
    </xh:body>
</xh:html>

Nice example of using a predicate to check the validity of inputs in a calculation!