% Page 134 of handout (p. 172 of book).
k([s,t,u,v]).
m(p, u).
m(p, v).
m(q, t).
m(q, v).
m(a, [t|v]).
m(a, [v|t]).
m(a, [s|u]).
m(a, [u|s]).
m(b, [u|v]).
m(b, [v|u]).
m(b, [s|t]).
m(b, [t|s]).