[Haskell-beginners] How do I use Guards in record syntax?

[Dimitri DeFigueiredo]

I'm having some trouble understanding where I can or cannot use guards 
inside record syntax. I'm writing a simple conversion routine, but I am 
not able to write it without inserting an extra let. Do I need a let 
expression here? Am I missing something?

--------------
data OldTrade = OldTrade {
     oldprice   :: Double   ,
     oldamount  :: Double   ,
     oldbuysell :: String  -- "buy", "sell" or ""
     } deriving( Eq, Show)


data BuyOrSell = Buy | Sell | Unknown deriving(Eq, Show)

data Trade = Trade {
     price   :: Double   ,
     amount  :: Double   ,
     buysell :: BuyOrSell
     } deriving( Eq, Show)

convert :: OldTrade -> Trade

convert ot  = Trade { price   = oldprice  ot,
                       amount  = oldamount ot,
                       buysell = let x | oldbuysell ot == "buy"  = Buy
                                       | oldbuysell ot == "sell" = Sell
                                       | otherwise               = Unknown
                                 in x
                     }

-- how do I eliminate the 'let' expression here?
-- I wanted to write something like:
--
--                      buysell | oldbuysell ot == "buy"  = Buy
--                              | oldbuysell ot == "sell" = Sell
--                              | otherwise               = Unknown

--------------

Thanks!

Dimitri