See the link in the last but one Example: Example: the Output Hierarchic Section Numbers,

https://www.w3.org/TR/2017/REC-xslt-30-20170608/#accumulator-examples
The code is as follows:

 
"Initial - value="https://bbs.csdn.net/topics/0" & gt; 
Select="0, the head ($value) + 1, tail ($value)"/& gt; 
The select="tail ($value) (: pop:)"/& gt; 
 

 
The separator="./"& gt; 
 
 

Excuse me, XSL: the second accumulator declaration & lt; XSL: an accumulator - rule> ": pop:" is what mean? Thank you