verbatimtex
\documentclass{article}
\begin{document}
etex;

defaultfont := "cmr12";
defaultscale := 12pt /fontsize defaultfont;
input mp-tool;
prologues := 2;
input boxes;

beginfig(0)
  boxit.xl( btex $x$ etex );
  xl.s  := (1cm,0);
  drawarrow xl.s - (0,1cm) -- xl.s;
  drawboxed( xl );
endfig;

end.