code: public boolean eq(Matrix B) {
          Matrix A = this;
          if (B.M != A.M || B.N != A.N) {
              throw new RuntimeException("Illegal matrix dimensions.");
          }
          for (int i = 0; i < M; i++) {
              for (int j = 0; j < N; j++) {
                  if (A.data[i][j] != B.data[i][j]) {
                      return false;
                  }
              }
          }
          return true;
      }
docs: does A = B exactly?