@type op() :: :eq | :neq | :gt | :gte | :lt | :lte | :in | :and | :or | :not

@type t() ::
  {:attr, key :: String.t() | atom(), op(), value :: any()}
  | {:ref, key1 :: String.t() | atom(), op(), key2 :: String.t() | atom()}
  | {:diff, key1 :: String.t() | atom(), key2 :: String.t() | atom(), op(),
     expected :: any()}
  | {:sum, keys :: [String.t() | atom()], op(), expected :: any()}
  | {:and, [t()]}
  | {:or, [t()]}
  | {:not, t()}
  | :always_true