code: public Matrix plus(Matrix B) {
          Matrix A = this;
          if (B.M != A.M || B.N != A.N) {
              throw new RuntimeException("Illegal matrix dimensions.");
          }
          Matrix C = new Matrix(M, N);
          for (int i = 0; i < M; i++) {
              for (int j = 0; j < N; j++) {
                  C.data[i][j] = A.data[i][j] + B.data[i][j];
              }
          }
          return C;
      }
docs: return C = A + B