svoid test_QuadTreeBitMatrix() {
  // 4*4, one pixel white
  BitSetMatrix m = new(4, 4);
  m.set(1, 1);
  QuadTreeBitMatrix qd = bitMatrixToQuadTree(m);
  assertTrueVerbose(equal := eqMatrices(m, qd));
  assertEqualsVerbose(2, qd.compositeNodeCount());
  
  // turn pixel back to black
  qd.set(1, 1, false);
  m.set(1, 1, false);
  assertTrueVerbose(equal := eqMatrices(m, qd));
  print(structure(qd));
  qd.set(1, 1, false);
  print(structure(qd));
  assertEqualsVerbose(0, qd.compositeNodeCount());
}