public String sellStock(Stock st, int numShares){
	portfolio = qs.qsort(portfolio, 0); // 0 sorts by ticker name, handy when  printing
	int index = -1;
	// check if they already own some shares in that stock
	String searchedTicker = st.getTicker();
	// traverse until ticker is found in portfolio
	for (int i = 0; i < portfolio.size(); i++){
	    StockPosition temp = portfolio.get(i);
	    String tempTicker = temp.getTicker();
	    if (tempTicker.equals(searchedTicker)){ // this is the index of that ticker in portfolio
		index = i;
		break;
	    }
	}
	if (index == -1)
	    return "Invalid stock";
	else if (portfolio.get(index).getNumShares() < numShares)
	    return "You do not own sufficient shares";
	else{
	    portfolio.get(index).sellShares(numShares);
	    money += st.getPrice() * numShares;
	    if (portfolio.get(index).getNumShares() <= 0)
		portfolio.remove(index);
	    return "Sold " + numShares + " shares";
	}
	
    }