interface Matrix<A> extends WidthAndHeight {
  public A get(int x, int y);
  public void set(int x, int y, A a);
  
  default Pt size() { ret pt(getWidth(), getHeight()); }
  default int nCells() { ret getWidth()*getHeight(); }
  
  default A get(Pt p) { ret get(p.x, p.y); }
  default void set aka put(Pt p, A a) { set(p.x, p.y, a); }
  
  // one row of the matrix as a virtual list (mutable)
  default L<A> getLine(int y) {
    ret new RandomAccessAbstractList<A>() {
      public int size() { ret getWidth(); }
      
      public A get(int x) {
        ret Matrix.this.get(x, y);
      }
      
      public A set(int x, A val) {
        A old = Matrix.this.get(x, y);
        Matrix.this.set(x, y, val);
        ret old;
      }
    };
  }
}