\def\lbrace{{{\setcount0=`}}}
\def\rbrace{{\setcount0=`{}}}
\tracingcommands1 \tracingonline1 \tracingmacros1
\hbox\lbrace x\rbrace
\showlists
\halign{a#b&c#d\cr
\lbrace&\rbrace\cr
\rbrace&\lbrace\cr
\showlists