message: Replace `A != NULL` with `HostReadData() != NULL`.
  NOTE: This is very innefficient.
diff: @@ -1496,7 +1496,8 @@ void SparseMatrix::Symmetrize()
  int SparseMatrix::NumNonZeroElems() const
  {
  - if (A != NULL) // matrix is finalized
  +
  + if (HostReadData() != NULL) // matrix is finalized
  {
  HostReadI();
  return I[height];
  