sol-2.24.tex

Here is the result printed by the interpreter:
\begtt\scm
(list 1 (list 2 (list 3 4)))
;Value: (1 (2 (3 4)))
\endtt
And here are the corresponding box-and-pointer structure, and its interpretation as a tree:
\medskip
\bgroup\typoscale[900/]
\centerline{%
    \hfil
    \directmetapost{
    beginfig(1);
    u = 0.7cm;
      z0 = (0, 0);
      z1 = (-u, -u);
      z2 = (0, -2u);
      z3 = (u, -3u);
      z4 = (3u, -3u);
      z5 = (2u, -2u);
      z6 = (u, -u);
      pickup pencircle scaled 4pt;
      drawdot z0;
      drawdot z5;
      drawdot z6;
      pickup pencircle scaled .8pt;
      draw z0--z1;
      draw z0--z4;
      draw z6--z2;
      draw z5--z3;
      label.llft(btex {\tt 1} etex, z1);
      label.llft(btex {\tt 2} etex, z2);
      label.llft(btex {\tt 3} etex, z3);
      label.lrt(btex {\tt 4} etex, z4);
      label.top(btex {\tt (1 (2 (3 4)))} etex, z0);
      label.urt(btex {\tt (2 (3 4))} etex, z6);
      label.urt(btex {\tt (3 4)} etex, z5);
    endfig;
    end
}}%
\egroup
\mnote{Add missing box-and-pointer structure!}