I have some code that I would like to use to append an edge to a Node data structure:

import Data.Set (Set)
import qualified Data.Set as Set

data Node = Vertex String (Set Node)
    deriving Show

addEdge :: Node -> Node -> Node
addEdge (Vertex name neighbors) destination
    | Set.null neighbors    = Vertex name (Set.singleton destination)
    | otherwise             = Vertex name (Set.insert destination neighbors)

However when I try to compile I get this error:

No instance for (Ord Node)
      arising from a use of `Set.insert'

As far as I can tell, Set.insert expects nothing but a value and a set to insert it into. What is this Ord?