vector * new (int n_bits)
{
    vector * p = malloc (sizeof (vector));
    if (! p) {
	croak ("Out of memory");
    }
    p->n_bits = n_bits;

    /* We use one char to store the bits. The C standard promises that
       one byte contains at least eight bits. */

    p->n_chars = (n_bits + 8 - 1) / 8;

    p->chars = calloc (p->n_chars, sizeof (unsigned char));

    if (! p->chars) {
	croak ("Out of memory");
    }
    return p;
}