>>> from filter_sts2 import filter_sts2

>>> L = [[77888999, ['B', 'A-', 'C-']], [22333444, ['B-', 'B+', 'A+']], [11222333, ['B', 'A', 'B+']]]

>>> Lsol = filter_sts2(L, 'B-')
>>> Lsol == [ [11222333, ['B', 'A', 'B+']], [22333444, ['B-', 'B+', 'A+']], [77888999, ['B', 'A-']]]
True

>>> Lsol = filter_sts2(L, 'B+')
>>> Lsol == [[11222333, ['A', 'B+']], [22333444, ['B+', 'A+']], [77888999, ['A-']]]
True

>>> Lsol = filter_sts2(L, 'A+')
>>> Lsol == [ [22333444, ['A+']] ]
True