\car
\def\@car#1#2\@nil{#1}
\cdr
\def\@cdr#1#2\@nil{#2}
\carcube
\def\@carcube#1#2#3#4\@nil{#1#2#3}
\@gobble
\long\def \@gobble #1{}
\long\def \@gobbletwo #1#2{}
\long\def \@gobblefour #1#2#3#4{}
\long\def\@firstofone#1{#1}
\long\def\@firstoftwo#1#2{#1}
\long\def\@secondoftwo#1#2{#2}
\let\@iden\@firstofone
\long\def\@thirdofthree#1#2#3{#3}