Yield, YieldFrame(not tested)

src/R3/Factories/Return.cs

@@ -1,7 +1,11 @@
-namespace R3;
+using System;
+
+namespace R3;
 
 public static partial class Observable
 {
+    // TODO: CancellationToken?
+
     public static Observable<T> Return<T>(T value)
     {
         return new ImmediateScheduleReturn<T>(value); // immediate
@@ -22,7 +26,7 @@ public static partial class Observable
             }
         }
 
-        return new Return<T>(value, dueTime, timeProvider); // use ITimer
+        return new Return<T>(value, dueTime.Normalize(), timeProvider); // use ITimer
     }
 
     // Optimized case
@@ -46,6 +50,22 @@ public static partial class Observable
     {
         return ReturnInt32.GetObservable(value); // -1~9 singleton
     }
+
+    // util
+
+    public static Observable<Unit> Yield()
+    {
+        return new ThreadPoolScheduleReturn<Unit>(default);
+    }
+
+    public static Observable<Unit> Yield(TimeProvider timeProvider)
+    {
+        if (timeProvider == TimeProvider.System)
+        {
+            return new ThreadPoolScheduleReturn<Unit>(default);
+        }
+        return new Return<Unit>(default, TimeSpan.Zero, timeProvider);
+    }
 }
 
 internal sealed class Return<T>(T value, TimeSpan dueTime, TimeProvider timeProvider) : Observable<T>

src/R3/Factories/ReturnFrame.cs

@@ -2,16 +2,6 @@
 
 public static partial class Observable
 {
-    public static Observable<Unit> ReturnUnitFrame(CancellationToken cancellationToken = default)
-    {
-        return ReturnFrame(Unit.Default, ObservableSystem.DefaultFrameProvider, cancellationToken);
-    }
-
-    public static Observable<Unit> ReturnUnitFrame(FrameProvider frameProvider, CancellationToken cancellationToken = default)
-    {
-        return ReturnFrame(Unit.Default, frameProvider, cancellationToken);
-    }
-
     public static Observable<T> ReturnFrame<T>(T value, CancellationToken cancellationToken = default)
     {
         return ReturnFrame(value, ObservableSystem.DefaultFrameProvider, cancellationToken);
@@ -41,6 +31,18 @@ public static partial class Observable
     {
         return new NextFrame(frameProvider, cancellationToken);
     }
+
+    // util
+
+    public static Observable<Unit> YieldFrame(CancellationToken cancellationToken = default)
+    {
+        return ReturnFrame(Unit.Default, ObservableSystem.DefaultFrameProvider, cancellationToken);
+    }
+
+    public static Observable<Unit> YieldFrame(FrameProvider frameProvider, CancellationToken cancellationToken = default)
+    {
+        return ReturnFrame(Unit.Default, frameProvider, cancellationToken);
+    }
 }
 
 internal sealed class ReturnFrame<T>(T value, FrameProvider frameProvider, CancellationToken cancellationToken) : Observable<T>