message: Correction in the non const version
diff: @@ -139,7 +139,7 @@ double& BlockMatrix::Elem (int i, int j)
  findGlobalRow(i, iblock, iloc);
  findGlobalCol(j, jblock, jloc);
  - if (IsZeroBlock(i, j))
  + if (IsZeroBlock(iblock, jblock))
  {
  mfem_error("BlockMatrix::Elem");
  }
  