diff --git a/Implab/Components/ExecutionState.cs b/Implab/Components/ExecutionState.cs
--- a/Implab/Components/ExecutionState.cs
+++ b/Implab/Components/ExecutionState.cs
@@ -13,6 +13,12 @@
 
         Running,
 
+        Suspending,
+
+        Suspended,
+
+        Resuming,
+
         Stopping,
 
         Failed,
diff --git a/Implab/PromiseExtensions.cs b/Implab/PromiseExtensions.cs
--- a/Implab/PromiseExtensions.cs
+++ b/Implab/PromiseExtensions.cs
@@ -180,8 +180,8 @@ namespace Implab {
         /// <param name="cleanup">A callback used to cleanup already resolved promises in case of an error</param>
         /// <returns></returns>
         public static IPromise<T[]> PromiseAll<T>(this ICollection<IPromise<T>> that, Action<T> cleanup) {
-            Safe.ArgumentNotNull(that, "that");
-            
+            Safe.ArgumentNotNull(that, "that");
+
             int count = that.Count;
 
             if (count == 0)
@@ -263,6 +263,8 @@ namespace Implab {
 
         public static IPromise<T2> Then<T, T2>(this IPromise<T> that, Func<T, T2> success, Func<Exception, T2> error, Func<Exception, T2> cancel) {
             Safe.ArgumentNotNull(that, "that");
+            Safe.ArgumentNotNull(success, "success");
+
             var d = new FuncTask<T, T2>(success, error, cancel, false);
             that.On(d.Resolve, d.Reject, d.CancelOperation);
             d.CancellationRequested(that.Cancel);
@@ -276,8 +278,8 @@ namespace Implab {
                     success(x);
                     return x;
                 },
-                error, 
-                cancel, 
+                error,
+                cancel,
                 false
             );
             that.On(d.Resolve, d.Reject, d.CancelOperation);
@@ -427,6 +429,13 @@ namespace Implab {
 
         #endregion
 
+        public static IPromise<T2> Guard<T, T2>(this IPromise<T> that, Func<IPromise<T>, IPromise<T2>> continuation, Action<T> cleanup) {
+            Safe.ArgumentNotNull(that, "that");
+            Safe.ArgumentNotNull(continuation, "continuation");
+            return continuation(that).Error((err) => {
+                that.On(cleanup);
+            }, true);
+        }
 
 #if NET_4_5
 
@@ -442,6 +451,24 @@ namespace Implab {
             return new PromiseAwaiter(that);
         }
 
+        public static IPromise BoundCancellationToken(this IPromise that, CancellationToken ct) {
+            Safe.ArgumentNotNull(that, "that");
+            ct.Register(that.Cancel);
+            return that.Then(null, null, (err) => {
+                ct.ThrowIfCancellationRequested();
+                throw new PromiseTransientException(err);
+            });
+        }
+
+        public static IPromise<T> BoundCancellationToken<T>(this IPromise<T> that, CancellationToken ct) {
+            Safe.ArgumentNotNull(that, "that");
+            ct.Register(that.Cancel);
+            return that.Then(null, null, (err) => {
+                ct.ThrowIfCancellationRequested();
+                throw new PromiseTransientException(err);
+            });
+        }
+
 #endif
     }
 }
diff --git a/Implab/Safe.cs b/Implab/Safe.cs
--- a/Implab/Safe.cs
+++ b/Implab/Safe.cs
@@ -127,5 +127,6 @@ namespace Implab
                 return Promise<T>.FromException(err);
             }
         }
+
     }
 }