This is my first attempt at creating a custom instance of a class such as Ord.

I've defined a new data structure to represent a list:

```
data List a = Empty | Cons a (List a)
    deriving (Show, Eq)

```

Now I want to define a new instance of Ord for List such that List a <= List b implies "the sum of the elements in List a is less than or equal to the sum of the elements in List b"

first of all, is it necessary to define a new "sum" function since the sum defined in Prelude wont work with the new List data type? then, how do I define the new instance of Ord for List?

thanks