def test_null():
assert ct_evens([]) == 0
def test_onenone():
assert ct_evens([1]) == 0
def test_repeats():
assert ct_evens([1, 2, 3, 2, 3, 3, 4, 2, 6, 6, 3, 9, 7, 4, 12]) == 8
def test_manynone():
assert ct_evens([1, 3, 5, 7, 9, 11, 13]) == 0
def test_allevens():
assert ct_evens([2, 4, 6, 8, 10, 12]) == 6
def test_manyevens():
assert ct_evens([2, 1, 5, 7, 8, 80]) == 3
def test_oneone():
assert ct_evens([42]) == 1
