[Haskell-cafe] A haskell error message

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 Kind regards, Ayawoa Sitsopé Dagbovie African Institute for Mathematical Sciences #6 Melrose Road Muizenberg 7945 South Africa. Phone: +27 782 960453