     /**
      Initializer that takes a collection of vertices with no edges.
 
-     - parameter vertices: A collection of vertice to include.
+     Sets all of their neighbor lists to the empty list.
+
+     - parameter vertices: A collection of vertices to include.
 
      - complexity: O(`vertices.count`)
      */

    public func addVertex(vertex: Vertex) throws {
        if let _ = adjacencyList[vertex] {
            // The Swift Programming Language book seems to prefer the pattern
            // `if <something> != nil` to this optional binding to the empty
            // pattern.
            throw GraphError.VertexAlreadyPresent
        }
        else {

    }

    /**
     A computed property of number of vertices currently in the graph.
     */
    private var size: Int {
        get {
