Execute the function. The function must return
a valid object.
XMLString s1 = m_arg0.execute(xctxt).xstr();
XMLString s2 = m_arg1.execute(xctxt).xstr();
int index = s1.indexOf(s2);
return (-1 == index)
? XString.EMPTYSTRING
: (XString)s1.substring(index + s2.length());