Where is the function FSNS

and can you provide a copy of the code?

http://opengrok.libreoffice.org/xref/core/include/sax/fshelper.hxx#FSNS

Thank you but I am trying to figure out how Writer handles underlines and overlines. Does this mean that it is actually done in FSend?

You have chosen wrong place to post your question. This kind of topics is best asked in #libreoffice-dev list on FreeNode IRC. And being on that channel is a must for anyone developing LibreOffice, anyway.