bal --flat food:out
<<<
~ Monthly
  Expenses:Auto:Fuel           $120.00
  Expenses:Food:Out            $100.00
  Expenses:Food:Groceries      $350.00
  Assets

2009/11/01
  Expenses:Food:Out             $50.00
  Assets
>>>1
              $50.00  Expenses:Food:Out
>>>2
=== 0
bal --flat --budget food:out
>>>1
             $-50.00  Expenses:Food:Out
>>>2
=== 0