F# value restriction

I have read all treads about value restriction in F#, but I still not understand it. I have the following code:

type tree<'a> = 
    | Nil
    | Node of (tree<'a> * 'a * tree<'a>)

let rec flatten = function
    | Nil -> []
    | Node ( Nil, b, Nil ) -> [b]
    | Node ( l, h, p ) -> List.concat [(flatten l);[h];(flatten p)]

and the compiler show an error:

error FS0030: Value restriction. The value 'it' has been inferred to have generic type
    val it : '_a list    
Either define 'it' as a simple data term, make it a function with explicit arguments or, if you do not intend for it to be generic, add a type annotation.

Can anyone help me? Thank you very much;)

Solution

Allow me to use my psychic debugging skills. You cannot call flatten Nil because, as the compiler indicates, the result could be an 'a list for any type 'a. You must add a type annotation, such as (flatten Nil : int list).

On an unrelated note, your second case in the definition of flatten is unnecessary and can be removed since it's also covered by the third case.