COMMENT ⊗   VALID 00002 PAGES
C REC  PAGE   DESCRIPTION
C00001 00001
C00002 00002
C00018 ENDMK
C⊗;

\magnification = \magstephalf
\catcode`|=13
\def|#1|{\hbox{\it#1\/}}
\parindent 0pt
\parskip 0pt
\catcode`⊗=13
\def⊗{\hbox to 2em{}}
\def\ppskip{\penalty-2000\vskip 10pt plus 3pt minus 2pt}
\def\IF{\mathop{\hbox{\bf if}}}
\def\THEN{\mathrel{\bf then}}
\def\ELSE{\mathrel{\bf else}}
\def\OR{\mathrel{\bf or}}
\def\N{\mathop{\hbox{\bf n}}}
\def\NIL{\hbox{\tt NIL}}
\def\.{\mathbin{.}}
\def\A{\mathop{\hbox{\bf a}}}
\def\D{\mathop{\hbox{\bf d}}}


$|alt|[u]←$\par
$⊗⊗⊗\IF\N u\OR\N\D u\THEN u$\par
$⊗⊗⊗\ELSE\A u\.|alt|[\D\D u]$\par

\ppskip
$|double|[u]←$\par
$⊗⊗⊗\IF\N u\THEN \NIL$\par
$⊗⊗⊗\ELSE\A u\.\A u\.|double|[\D u]$\par

\vfill\end