         return nil
     }
     
+    /// `true` iff `self` contains `node`.
+    ///
+    /// - Complexity: O(`count of nodes`).
     @warn_unused_result
     public func contains(node: Node) -> Bool {
         for (_node, list) in table where _node == node || list.keys.contains(node) {

        return table[fromNode]!.updateValue(link, forKey: toNode)
    }
    
    public mutating func removeLink(from fromNode: Node, to toNode: Node) -> Link? {
        if var list = table[fromNode], let result = list[toNode] {
            list.removeValueForKey(toNode)

    }
    
    /// A set containing just the nodes of `self`.
    ///
    /// - Complexity: O(`count of nodes`).
    public var nodes: Set<Node> {
        return graph.nodes
    }
