static <A> L<A> colToList(Matrix<A> m, int x) {
  if (m == null) null;
  rangeCheck(x, m.getWidth());
  int h = m.getHeight();
  
  ret new RandomAccessAbstractList<A>() {
    public int size() { ret h; }
    
    public A get(int y) {
      ret m.get(x, y);
    }
    
    public A set(int y, A val) {
      A old = m.get(x, y);
      m.set(x, y, val);
      ret old;
    }
  };
}