operator precendence grammar
I think that answer is clear (although of course I would do, because I wrote it).

What it says is:


  if you find


$$nonterminal\; TERMINAL$$


  in any production, then you add the precedence relations $TRAIL \gtrdot TERMINAL$ for every $TRAIL$ in $Trailing(nonterminal)$. Similarly, every occurrence of


$$TERMINAL\; nonterminal$$


  generates the relationships $TERMINAL \lessdot LEAD$ for every $LEAD$ in $Leading(nonterminal)$.


So in $'(' expr ')'$, you have both an instance of $TERMINAL\; nonterminal$ and an instance of $nonterminal\; TERMINAL$, and you need to deal with both of them independently.