docs: public static double[][] plusEquals(final double[][] m1, final double[][] m2) {
      final int rowdim = m1.length, coldim = getColumnDimensionality(m1);
      assert getRowDimensionality(m1) == getRowDimensionality(m2) && coldim == getColumnDimensionality(m2) : ERR_MATRIX_DIMENSIONS;
      for(int i = 0; i < rowdim; i++) {
        final double[] row1 = m1[i], row2 = m2[i];
        for(int j = 0; j < coldim; j++) {
          row1[j] += row2[j];
        }
      }
      return m1;
    }