code: public static double[][] minusEquals(final double[][] m1, final double[][] m2) {
      final int columndimension = getColumnDimensionality(m1);
      assert getRowDimensionality(m1) == getRowDimensionality(m2) && columndimension == getColumnDimensionality(m2) : ERR_MATRIX_DIMENSIONS;
      for(int i = 0; i < m1.length; i++) {
        final double[] row1 = m1[i], row2 = m2[i];
        for(int j = 0; j < columndimension; j++) {
          row1[j] -= row2[j];
        }
      }
      return m1;
    }
docs: Component-wise matrix subtraction: m1 = m1 - m2,
  overwriting the existing matrix m1.
  
  @param m1 Input matrix
  @param m2 another matrix
  @return m1 - m2, overwriting m1