     /**
       Initiates a GET request to update the state of this resource, unless it is already up to date.
       
-      “Up to date” means the following:
-      
-      - either
-          - the resource has data (i.e. `latestData` is not nil),
-          - the last request succeeded (i.e. `latestError` _is_ nil), and
-          - the timestamp on `latestData` is more recent than `expirationTime`,
-      - or
-          - the last request failed (i.e. `latestError` is not nil), and
-          - the timestamp on `latestError` is more recent than `retryTime`
+      “Up to date” means that either:
+        - the resource has data (i.e. `latestData` is not nil),
+        - the last request succeeded (i.e. `latestError` _is_ nil), and
+        - the timestamp on `latestData` is more recent than `expirationTime` seconds ago,
+      
+      …or:
+        - the last request failed (i.e. `latestError` is not nil), and
+        - the timestamp on `latestError` is more recent than `retryTime` seconds ago.
     
       If the resource is not up to date, this method calls `load()`.
     */

    // MARK: URL Navigation
    
    /**
      Returns the resource with the given string appended to this resource’s URL, with a joining slash inserted if
      necessary.
     
      Use this method for hierarchical resource navigation. The typical use case is constructing a resource URL from
      path components and IDs:

    {
    // MARK: Configuration
    
    /// The API to which this resource belongs. Provides configuration defaults and instance uniqueness.
    public let service: Service
    
    /// The canoncial URL of this resource.
    public let url: NSURL? // TODO: figure out what to do about invalid URLs
    
    internal var observers = [ObserverEntry]()
