static void
incr (char **s0, size_t *s_len)
{
char *s = *s0;
char *endp = s + *s_len - 1;

do
{
if ((*endp)++ < '9')
return;
*endp-- = '0';
}
while (endp >= s);
*--(*s0) = '1';
++*s_len;
}