static <A> Matrix<A> listOfListsToMatrix(LL<A> data) {
  if (data == null) null;
  int _h = l(data);
  if (_h == 0) null;
  int _w = l(data.get(0));

  class ListsToMatrix extends AbstractMatrix<A> {
    *() { super(_w, _h); }
  
    public A get(int x, int y) { ret data.get(y).get(x); }
    public void set(int x, int y, A a) { data.get(y).set(x, a); }
  };
  
  ret new ListsToMatrix;
}

Began life as a copy of #1033120