>>> from filter_sts import filter_sts

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

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

>>> Lsol = filter_sts(L, 'B')
>>> Lsol == [ [11222333, ['B','A','B+']] ]
True

>>> Lsol = filter_sts(L, 'B+')
>>> Lsol == [  ]
True