How can I do that in a browser enabled form? Normally a trivial XPath expression like "mynode[1]" or "mynode[number()=1]" or "mynode[position()=1]" does the trick - but no luck there: the former two get an "The following expression could not be parsed because of a syntax error or
because it uses an undefined namespace prefix or unsupported function" during the verification stage (but work fine in the client), the latter is marked as unsupported right in the "insert formula" menu.