message: Force using const version to prevent error when accessing undefined terms
diff: @@ -159,8 +159,8 @@ const double& BlockMatrix::Elem (int i, int j) const
  {
  mfem_error("BlockMatrix::Elem");
  }
  -
  - return Aij(iblock, jblock)->Elem(iloc, jloc);
  + const SparseMatrix& AijSM = *Aij(iblock, jblock);
  + return AijSM.Elem(iloc, jloc);
  }
  int BlockMatrix::RowSize(const int i) const
  