'set bit number bitno to true'

def set(self, bitno):

    (byteno, bit_within_byteno) = divmod(bitno, 8)
    mask = (1 << bit_within_byteno)
    self._mmapped_file.seek(byteno)
    char = self._mmapped_file.read(1)
    byte = ord(char)
    byte |= mask
    self._mmapped_file.seek(byteno)
    self._mmapped_file.write(chr(byte))
