File:Kripke-1.png

Summary

Metapost source:

input boxes;

verbatimtex
\newdimen\w
\setbox0=\hbox{$p, q$}
\w=\wd0
\def\h#1{\strut \hbox to\w{\hfil#1\hfil}}
etex

def arrow(suffix $, $$) = 
  drawarrow $.c..$$.c cutbefore bpath.$ cutafter bpath.$$
enddef;

def loop(suffix $) =
drawarrow $.c{dir(45)}..2[$.c,$.n]..{dir(-45)}$.c
  cutbefore bpath.$ cutafter bpath.$
enddef;
  
numeric u; u = 60pt;

beginfig(1)
circleit.a1(btex \h{$q$} etex);
circleit.a2(btex \h{} etex);
circleit.a3(btex \h{$p, q$} etex);
a2.c - a1.c = a3.c - a2.c = (u, 0);
drawboxed(a1, a2, a3);
arrow(a1, a2); arrow(a2, a3);
loop(a1); loop(a3);
label.bot(btex $s_0$ etex, a1.s);
label.bot(btex $s_1$ etex, a2.s);
label.bot(btex $s_2$ etex, a3.s);
endfig;

end

I really love the way the arrowheads on curved lines are themselves slightly curved.