/usr/share/doc/frown-doc/examples/manual/Paren2.lg is in frown-doc 0.6.1-14.
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 | > module Paren2 where
> import Result
>
> data Tree = Leaf | Fork Tree Tree
> deriving (Show)
>
> %{
>
> Terminal = '(' | ')';
>
> Nonterminal = paren {Tree};
>
> paren {Leaf} : ;
> {Fork t u} | paren {t}, '(', paren {u}, ')';
>
>
> }%
>
> frown _ = fail "syntax error"
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