opener :: Lexer (Pos Token)
opener = withPos $ choice [ const OpenParen <$> char '('
                          , const OpenBrack <$> char '['
                          , const OpenBrace <$> char '{'
                          ] <* disable
opener :: Lexer (Pos Token)
opener = withPos $ choice [ const OpenParen <$> char '('
                          , const OpenBrack <$> char '['
                          , const OpenBrace <$> char '{'
                          ] <* disable
opener = withPos $ choice [ const OpenParen <$> char '('
                          , const OpenBrack <$> char '['
                          , const OpenBrace <$> char '{'
                          ] <* disable