更新异步step

DoRunner/BackpointRunner.cs

@@ -1,4 +1,5 @@
 using Convention.RScript.Parser;
+using System.Collections;
 using System.Diagnostics.CodeAnalysis;
 
 namespace Convention.RScript.Runner
@@ -27,5 +28,23 @@ namespace Convention.RScript.Runner
             }
             return null;
         }
+
+        [return: MaybeNull]
+        public override IEnumerator RunAsync(ExpressionParser parser, RScriptSentence sentence, RScriptContext context)
+        {
+            // 检查并跳转到上次跳转的位置
+            if (parser.Evaluate<bool>(sentence.content))
+            {
+                if (context.GotoPointerStack.Count == 0)
+                {
+                    throw new RScriptRuntimeException($"No position to back.", context.CurrentRuntimePointer);
+                }
+                else
+                {
+                    DoJumpRuntimePointer(parser, context.GotoPointerStack.Pop(), context);
+                }
+            }
+            yield break;
+        }
     }
 }