def test_concatenation_ab(a, b):
    concAB = (a + b)
    assert (not concAB.accepts(''))
    assert (not concAB.accepts('a'))
    assert (not concAB.accepts('b'))
    assert (not concAB.accepts('aa'))
    assert concAB.accepts('ab')
    assert (not concAB.accepts('ba'))
    assert (not concAB.accepts('bb'))
def test_concatenation_ab(a, b):
    concAB = (a + b)
    assert (not concAB.accepts())
    assert (not concAB.accepts('a'))
    assert (not concAB.accepts('b'))
    assert (not concAB.accepts('aa'))
    assert concAB.accepts('ab')
    assert (not concAB.accepts('ba'))
    assert (not concAB.accepts('bb'))