.. efun:: int next_bit (string str, int start)
  int next_bit (string str, int start, int find_cleared)

  Return the number of the next bit in bitstring :arg:`str` after position :arg:`start`. Usually this is the next set bit, but if :arg:`find_cleared` is given and not 0, the position of the next cleared bit is returned.

  Note that finding cleared bits after the last set bit is limited to the actual length of :arg:`str`.

  Each character contains 6 bits. So you can store a value between 0 and 63 in one character (2^6=64). Starting character is the blank :char:`SPACE` which has the value 0. The first character in the string is the one with the lowest bits (0-5).

  .. usage::

    The below will write ``2`` and ``4``::

      string s;
      int p;

      s = set_bit("", 4); s = set_bit(s, 2);

      for (p = -1; -1 != (p = next_bit(s, p); )
          write(p+"\n");

  .. seealso:: :efun:`set_bit`, :efun:`clear_bit`, :efun:`test_bit`, :efun:`last_bit`, :efun:`count_bits`, :efun:`and_bits`, :efun:`or_bits`, :efun:`xor_bits`, :efun:`invert_bits`, :efun:`copy_bits`