unknown function: 'string-value'

I use Quip from my Java application, with using the quip.jar and quip.exe

my request is
"for $d IN document(".\data\EssaiMapFolder\infos.xml")//documents return string-value($d/title)"

and the response is
quip:message<![CDATA[Error (1,73): unknown function: 'string-value'

please help me

The function is now simply called “string”.
Quip can list you the implemented functions both from the command line and from the GUI.