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

    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 {

    /**
     Initializer that takes a collection of vertices with no edges.

     Sets all of their neighbor lists to the empty list.

     - parameter vertices: A collection of vertices to include.

     - complexity: O(`vertices.count`)
     */
