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

• Why do we need the option id, anyway?

%siblings{};       # exactly the same as %siblings, except

say "%siblings";   # "%siblings"
say "%siblings{}"; # charlie   => donald etc.