def test_PropKB():
A, B, C = symbols('ABC')
kb = PropKB()
kb.tell(A >> B)
kb.tell(B >> C)
assert kb.ask(A) == True
assert kb.ask(B) == True
assert kb.ask(C) == True
assert kb.ask(~A) == True
assert kb.ask(~B) == True
assert kb.ask(~C) == True
kb.tell(A)
assert kb.ask(A) == True
assert kb.ask(B) == True
assert kb.ask(C) == True
assert kb.ask(~C) == False
kb.retract(A)
assert kb.ask(~C) == True
kb2 = PropKB(Equivalent(A, B))
assert kb2.ask(A) == True
assert kb2.ask(B) == True
kb2.tell(A)
assert kb2.ask(A) == True
kb3 = PropKB()
kb3.tell(A)
def test_propKB_tolerant():
""""tolerant to bad input"""
kb = PropKB()
A, B, C = symbols('ABC')
assert kb.ask(B) == False