[Zope] using a variable plus attribute inside a DTML-CALL

[Mitchell Model]

I am constructing a URL for a redirection based on a variable 'arg' in the REQUEST.  I can use that variable in a DTML-CALL, as follows (helped by discussions I found at NIP from a while ago in this list, e.g. http://zope.nipltd.com/public/lists/zope-archive.nsf/242bb7cc2b2c343d802568ab003585d4/54b2a6e5a68d088980256982004fea79?OpenDocument).

<dtml-call
expr="RESPONSE.redirect('http://www.zope.org/SiteIndex/search?text_content='
                        + arg)">

However, I want to url_quote_plus the arg.  The usual variable attribute syntax (either <dtml-var arg.url_quote_plus> or &dtml.url_quote_plus.arg;) isn't available inside the Python expression in the dtml-call.  Without creating an external method to do the quoting, how can I get the quoted version of the variable inside the Python expression?  I've tried lots of different ideas, but none worked.