!python
def foo(d, val, x):
    return d.xact.amount + val
!end

--amount-expr 'foo'({$100}, 20)

2004/05/01 Checking balance
  Assets:Checking               $500.00
  Equity:Opening Balances

2004/05/29 Book Store
  Expenses:Books                 $20.00
  Assets:Checking

2004/05/29 Restaurant
  Expenses:Food                  $50.00
  Liabilities:MasterCard