'Return true iff bit number bitno is set'

def is_set(self, bitno):

    (byteno, bit_within_wordno) = divmod(bitno, 8)
    mask = (1 << bit_within_wordno)
    self._mmapped_file.seek(byteno)
    char = self._mmapped_file.read(1)
    byte = ord(char)
    return (byte & mask)
