If this ParserExtension is registered with a parser and the parser
encounters the name of the function in the string it is parsing,
then the parser will call this routine.
When the name of this ParserExtension is encountered by a parser with which
the extension is registered, the parser calls this routine to parse the
summation subexpression.
Create a string that is displayed at the reference point (xPos,yPos);
The positioning constant, pos, gives the positioning relative to this point, if xPos or yPos is non-null.