static <A> Matrix<A> listToMatrix(int _w, L<A> data) {
  int n = l(data);
  int _h = n/_w;
  if (_w*_h != n) fail("Bad size", +n, +_w, +_h);
  
  class ListToMatrix extends AbstractMatrix<A> {
    *() { super(_w, _h); }
    
    int xyToIndex(int x, int y) {
      ret y*w+x;
    }
  
    public A get(int x, int y) { ret data.get(xyToIndex(x, y)); }
    public void set(int x, int y, A a) { data.set(xyToIndex(x, y), a); }
  };
  
  ret new ListToMatrix;
}