def test_antiparallel_c():
    assert antiparallel('C') == 'G'
def test_antiparallel_t():
    assert antiparallel('T') == 'A'
def test_antiparallel_a():
    assert antiparallel('A') == 'T'
def test_antiparallel_empty_string():
    assert antiparallel('') == ''
def test_antiparallel_gtca():
    assert antiparallel('GTCA') == 'TGAC'
def test_antiparallel_gggg():
    assert antiparallel('GGGG') == 'CCCC'
def test_antiparallel_g():
    assert antiparallel('G') == 'C'