  try (parens expr)
       <|> try (Top <$ choice [reserved "True", reserved "⊤"])
       <|> try (Bot <$ choice [reserved "False", reserved "⊥"])
       <|> try (Prop <$> upperIdentifier)
       <?> "simple expression"

term :: Parser L
term    =  try (parens expr)
       <|> try (Top <$ choice [reserved "True", reserved "⊤"])
       <|> try (Bot <$ choice [reserved "False", reserved "⊥"])
       <|> try (Prop <$> upperIdentifier)
       <?> "simple expression"
term    =  try (parens expr)
       <|> try (Top <$ choice [reserved "True", reserved "⊤"])
       <|> try (Bot <$ choice [reserved "False", reserved "⊥"])
       <|> try (Prop <$> upperIdentifier)
       <?> "simple expression"