def kb():
kb = Knowledge()
l1 = lit(pred('y', 1), [var('X')])
l2 = lit(pred('x', 2), [var('X'), var('Y')])
l3 = lit(pred('z', 1), [var('Y')])
c1 = clause(l1, [l2, l3])
kb.assert_clause(c1)
assert kb.db
kb.retract_clause(c1)
assert not kb.db
