code: public Matrix plusEquals(Matrix B)
      {
          checkMatrixDimensions(B);
          for (int i = 0; i < m; i++)
          {
              for (int j = 0; j < n; j++)
              {
                  A[i][j] = A[i][j] + B.A[i][j];
              }
          }
          return this;
      }
docs: A = A + B
  
  @param B another matrix
  @return A + B