! calculates the opposite value of an expression as in:

• Expression: !<exp>
• Operand: expression (true, false)

Example: find 10 cards whose balances are 0

find().nodes({@card && !balance}) as n
limit 10
return n{*}

Analysis: the filter can not be put into {!@card.balance}, as the result would be returned as {!@card || !balance}'s otherwise.