解离出Runner

DoRunner/BackpointRunner.cs

@@ -0,0 +1,26 @@
+using Convention.RScript.Parser;
+using System.Diagnostics.CodeAnalysis;
+
+namespace Convention.RScript.Runner
+{
+    public class BackpointRunner : JumpRuntimePointerRunner
+    {
+        [return: MaybeNull]
+        public override object Run(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);
+                }
+            }
+            return null;
+        }
+    }
+}