code: public Matrix3D plus(Matrix3D B) {
          Matrix3D A = this;
          if (B.M != A.M || B.N != A.N || B.K != A.K) throw new RuntimeException("Illegal matrix dimensions.");
          Matrix3D C = new Matrix3D(M, N, K);
          for (int i = 0; i < M; i++)
              for (int j = 0; j < N; j++)
              	for (int l = 0; l < K; l++)
              		C.data[i][j][l] = A.data[i][j][l] + B.data[i][j][l];
          return C;
      }
docs: return C = A + B