Hi,
I have been trying to specify the depth-first numbering algorithm as follow:

>_dfNum             :: Int -> [Tree a] -> [Tree Int]
>_dfNum i ((Node n ts):us) = (Node i vs):(_dfNum (i+1+(length ts)) us) where
>    vs=_dfNum (i+1) ts

But when I test it, I receive the message:

Non-exhaustive patterns in function _bfNum

Please, could you give me an idea of how I could fix this?
Thanks