class List_double_3d_ {
    public double [3d] car;
    public List_double_3d_ cdr;

    // Main constructor.
    public List_double_3d_(double [3d] x, List_double_3d_ rest) {
	car = x;
	cdr = rest;
    }

    public List_double_3d_(double [3d] x) {
	car = x;
	cdr = null;
    }

    public List_double_3d_ cons(double [3d] x) {
	return new List_double_3d_(x, this);
    }        

    public inline double [3d] first() {
	return car;
    }
    public inline List_double_3d_ rest() {
	return cdr;
    }

    // Destructive append.
    public List_double_3d_ concat(List_double_3d_ l) {
      if (l != null)
	for (List_double_3d_ q = this; ; q = q.cdr)
	  if (q.cdr == null) {
	    q.cdr = l;
	    break;
	  }
      return this;
    }

    public int length() {
	int i = 0;
	List_double_3d_ r = this;
	while (r != null) {
	    i++;
	    r = r.rest();
	}
	return i;
    }
}