code: public SparseVector plus(SparseVector b) {
  		SparseVector a = this;
  		if (a.N != b.N) throw new IllegalArgumentException("Vector lengths disagree : " + a.N + " != " + b.N);
  		SparseVector c = new SparseVector(N);
  		for (int i : a.symbolTable) c.put(i, a.get(i));                // c = a
  		for (int i : b.symbolTable) c.put(i, b.get(i) + c.get(i));     // c = c + b
  		return c;
  	}
docs: Calcualtes return a + b
  
  @param b
  @return