素敵な \expandafter に巡り会えますように！
Happy \expandafter \expandafter\expandafter\expandafter{}\def\expandafTer%
#1#2#3#4#5#6#7{#4#5##1##2##3##4##5{##2##3##4##5##1}#4#2##1##2{#4##1####1{##2}}%
#4#3##1##2##3##4##5{##5##1##2##3##4}#4#1##1##2{##2##1}#4#6{}#4#7##1{#6}#7#7#6}%
\expandafter\expandafter{}\expandafTer\eXpandafter\expanDafter\expandafteR\def%
\exPandafter\Expandafter \expandaftEr\Expandafter\expanDafter\expAndafter ^^63%
{\expandaftEr \expandafter {}\Expandafter}\expanDafter \expandAfter ^^6f\def%
\eXpandaftEr {\expandafter \eXpandafter\expandafter\eXpandAfter\expandaftEr%
}\expandaftEr\Expandafter \expandaftEr\expandafter\expandaftEr\Expandafter\def%
\eXpandAfter{\expandafter \expandafter\expandafter \eXpandafter \expandafter%
\expandafter\expandafter \expandAftEr\expandafter\expandaftEr\expandafteR}\def%
\expandAftEr{\expandafter\expandafter\expandafter \eXpandafter \expandafter%
\expandafter\expandafter\eXpAndafter\expandafter\expandaftEr \expandafteR}\def%
\eXpAndafter{\expandafter \expandafter\expandafter\eXpandafter\expandafter%
\expandafter\expandafter\eXpAndaftEr\expandafter\expandaftEr \exPandafter}\def%
\eXpAndaftEr{\expandafter\eXpandafter\expandafter\eXpAndAfter\expandafteR}\def%
\eXpAndAfter{\expandafter\eXpandafter\expandafter\eXpAndAftEr\expandafteR}\def%
\eXpAndAftEr {\expandafter \expandafter \expandafter \expandafter \expandafter%
\expandafter\expandafter\expandafteR\expandafter \expandafter \expandafter %
\expAndafter\expandafter \expandafteR \expandAfter}\expandafter\eXpandafter%
\expandafter \eXpandaftEr\expandaftEr\\expandafter\Expandafter 12^^21\bye%
