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'