Seek an XPath expression which concatenates an arbitra

Hi Folks,

Eek! A basic question...

I have an XML document containing an arbitrary number of <binary-value>
elements:

<Document>
    <binary-value>0100</binary-value>
    <binary-value>11</binary-value>
    <binary-value>1010001</binary-value>
    ...
</Document>

I want an XPath expression which concatenates the binary values into one long
string:

0100111010001...

I thought this would work:

concat(for $i in //binary-value return $i)

but that gives an error, "Cannot find a 1-argument function named concat"

What is the correct XPath, please?

/Roger