>>> import re
>>> pieces = 'KQRRBBNN'
>>> bish = re.compile(r'B(|..|....|......)B').search
>>> king = re.compile(r'R.*K.*R').search
>>> starts3 = {p for p in (''.join(q) for q in permutations(pieces))
            if bish(p) and king(p)}
>>> len(starts3)
960
>>> starts3.pop()
'QRNKBNRB'
>>>