SYNOPSIS
        string invert_bits(string str)

DESCRIPTION
        Invert the status of all bits in bitstring <str> and return the
        new string.

        Note that the total number of bits (ie the string length) stays
        the same.

EXAMPLES
        string s;

        s = set_bit("", 3); s = set_bit(s, 4); s = set_bit(s, 15);
          --> s is now  "8 ("

        invert_bits(s) --> returns "G_W"

SEE ALSO
        set_bit(E), clear_bit(E), test_bit(E), last_bit(E), and_bits(E),
        or_bits(E), xor_bits(E), count_bits(E), copy_bits(E)