[Haskell-cafe] Patterns overlapped?

Hi, When I compile this code, ghc complains about some overlapped patterns in function depth. What on Earth is ghc talking about? O:-) data BinTree a = EmptyTree | NodeBT a (BinTree a) (BinTree a) deriving Show emptyBT = EmptyTree depth emptyBT = 0 depth (NodeBT _ left right) = max (1 + depth left) (1 + depth right)