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ruleHashSubscriptBraces :: RuleParser (Exp -> Exp)
ruleHashSubscriptBraces = do
    between (symbol "{") (char '}') $ option id $ do
        exp <- ruleExpression; return $ \x -> Syn "{}" [x, exp]

firstPossibleReturnValue = id
    where
    id x = x    -- actually, this is a standard function
otherPossibleReturnValue = \x -> Syn "{}" [x, exp]