I'm trying to write
$\sup' S$ — that is to say,
$\sup S$ but with a prime on the sup. However, in display style, this comes out instead with the prime on top of the sup instead of in the superscript position. Presumably this is because LaTeX reads it as a superscript, and in display style, superscripts go on top of
\sup just as subscripts go below it. How can I actually write
sup' S and have it come out right?