code: public SparseSquareMatrix plus(SparseSquareMatrix B) {
  
  		SparseSquareMatrix A = this;
  
  		if (A.N != B.N) throw new IllegalArgumentException("Dimensions disagree. " + A.N + " != " + B.N);
  
  		SparseSquareMatrix C = new SparseSquareMatrix(N);
  
  		for (int i = 0; i < N; i++)
  			C.rows[i] = A.rows[i].plus(B.rows[i]);
  
  		return C;
  	}
docs: return C = A + B
  
  @param B
  @return