% I just wanted to verify \sup was in plain TeX rather than just
% LaTeX.

% https://tex.stackexchange.com/questions/233832/how-to-align-braces-with-middle-vertical-line-the-same-size-in-math-mode
% suggests using \Bigl, \Biggl, etc., to get the right size of
% vertical bar, rather than \left and \right.  eTeX has \middle
% instead.

$$\sup \Biggl\{{-1 \over n} \Biggm| n \in 1, 2, 3, \cdots\Biggr\} = 0$$

\bye
