2014/06/01 Deposit some cash
  Equity:OpeningBalances
  Assets:Investing:Cash        $20000

2014/06/02 Buy one lot
  Assets:Investing:Stocks       10 MSFT @ $500
  Assets:Investing:Cash

2014/06/02 Buy another lot
  Assets:Investing:Stocks       10 MSFT @ $510
  Assets:Investing:Cash

2014/06/03 Sell at average cost
  Assets:Investing:Stocks      -10 MSFT {$505} @ $520
  Assets:Investing:Cash