Determine if the given parameter is a valid (Dutch) social security number, known as Burger Service Nummer.

gistfile1.py

def is_eleven_proof(bsn):
    """ Determine if the given parameter is a valid (Dutch) social security 
        number, known as Burger Service Nummer.
        The parameter may be an integer or a string. Non-digit characters will
        be filtered out.
        The function returns 'True' if the number is valid and 'False'
        otherwise.
    """

    if isinstance(bsn, (str, unicode)):
        bsn = "".join(n for n in bsn if n.isdigit())
    elif isinstance(bsn, int):
        bsn = str(bsn)
    else:
        return False

    weights = [-1, 2, 3, 4, 5, 6, 7, 8, 9]
    
    if len(bsn) not in (8,9):
        return False

    accum = 0
    for i, n in enumerate(bsn[::-1]):
        accum += weights[i] * int(n)

    if accum % 11:
        return False
    else: 
        return True