>>> from filter_sts import filter_sts

>>> L = [[77, ['B','A+','C-']], [22, ['B-','B+','A+']], [11, ['B','A','B+']]]

>>> Lsol = filter_sts(L, 'C-')
>>> Lsol == [[11, ['B', 'A', 'B+']], [22, ['B-','B+','A+']], [77, ['B','A+','C-']]]  # doctesttag: +TAG=1_filter_sts
True

>>> Lsol = filter_sts(L, 'C')
>>> Lsol == [[11, ['B','A','B+']], [22, ['B-','B+','A+']]]  # doctesttag: +TAG=1_filter_sts
True

>>> Lsol = filter_sts(L, 'B-')
>>> Lsol == [[11, ['B','A','B+']], [22, ['B-','B+','A+']]]  # doctesttag: +TAG=1_filter_sts
True

>>> Lsol = filter_sts(L, 'B')
>>> Lsol == [[11, ['B','A','B+']]]  # doctesttag: +TAG=1_filter_sts
True

>>> filter_sts(L, 'B+')   # doctesttag: +TAG=1_filter_sts
[]

>>> filter_sts(L, 'A')  # doctesttag: +TAG=1_filter_sts
[]