tests/examples/bidipath.txt

path(a, a).

path(a, a).
path(a, d).
path(a, c).
path(a, b).
path(b, b).
path(b, a).
path(b, d).
path(b, c).
path(c, c).
path(c, b).
path(c, a).
path(c, d).
path(d, d).
path(d, c).
path(d, b).
path(d, a).