message: improve sparsity assertions
diff: @@ -215,10 +215,12 @@ template <typename MatrixData1, typename MatrixData2>
  !std::equal(fst_row_begin, fst_row_end, snd_row_begin, col_eq)) {
  auto fail = ::testing::AssertionFailure();
  fail << "Sparsity pattern differs between " << first_expression
  - << " and " << second_expression << "\n\tIn row " << row << " "
  - << first_expression << " has columns:\n";
  + << " and " << second_expression << "\nIn row " << row << " "
  + << first_expression << " has " << (fst_row_end - fst_row_begin)
  + << " columns:\n";
  detail::print_columns(fail, fst_row_begin, fst_row_end);
  - fail << " and " << second_expression << " has columns:\n";
  + fail << "and " << second_expression << " has "
  + << (snd_row_end - snd_row_begin) << " columns:\n";
  detail::print_columns(fail, snd_row_begin, snd_row_end);
  return fail;
  }
  