c Example input for the vtree drawing in README.md.
c
c Twelve variables in three groups of four. Each group picks exactly one of
c its variables -- one at-least-one clause and the six at-most-one pairs --
c and four further clauses forbid a pick on both sides of a group boundary.
p cnf 12 25
1 2 3 4 0
-1 -2 0
-1 -3 0
-1 -4 0
-2 -3 0
-2 -4 0
-3 -4 0
5 6 7 8 0
-5 -6 0
-5 -7 0
-5 -8 0
-6 -7 0
-6 -8 0
-7 -8 0
9 10 11 12 0
-9 -10 0
-9 -11 0
-9 -12 0
-10 -11 0
-10 -12 0
-11 -12 0
-1 -5 0
-4 -8 0
-5 -9 0
-8 -12 0