diff --git a/Sources/SynKit/Benchmarks/Parser.Benchmarks/ExpressionBenchmarks.cs b/Sources/SynKit/Benchmarks/Parser.Benchmarks/ExpressionBenchmarks.cs
index 711be5e2..17b87a7d 100644
--- a/Sources/SynKit/Benchmarks/Parser.Benchmarks/ExpressionBenchmarks.cs
+++ b/Sources/SynKit/Benchmarks/Parser.Benchmarks/ExpressionBenchmarks.cs
@@ -29,6 +29,15 @@ public ParseResult<int> WorstManualExpressionParser()
     {
         return new WorstManualExpressionParser(new Lexer(source)).ParseProgram();
     }
+    public ParseResult<int> ComplexExpressionParser()
+    {
+        return new ComplexExpressionParser(new Lexer("((((((((((2)))))))));")).ParseProgram();
+    }
+
+    public ParseResult<int> ManualComplexExpressionParser()
+    {
+        return new ManualComplexExpressionParser(new Lexer("((((((((((2)))))))));")).ParseProgram();
+    }
 
     [Benchmark]
     public List<Token<TokenType>> Lex()
diff --git a/Sources/SynKit/Benchmarks/Parser.Benchmarks/ExpressionParser.cs b/Sources/SynKit/Benchmarks/Parser.Benchmarks/ExpressionParser.cs
index 922d7b62..7f0de6cd 100644
--- a/Sources/SynKit/Benchmarks/Parser.Benchmarks/ExpressionParser.cs
+++ b/Sources/SynKit/Benchmarks/Parser.Benchmarks/ExpressionParser.cs
@@ -25,6 +25,11 @@ public enum TokenType
     [Token("%")] Mod,
     [Token("^")] Exp,
 
+    [Token("?")] Conditional,
+    [Token(":")] Colon,
+    [Token(",")] Comma,
+    [Token("=")] Assignment,
+
     [Token(";")] Semicol,
 
     [Regex(Regexes.IntLiteral)] IntLit,
@@ -170,3 +175,68 @@ public partial class WorstManualExpressionParser
     [Rule("expression_atomic : IntLit")]
     public static int IntLit(IToken token) => int.Parse(token.Text);
 }
+
+[Parser(typeof(TokenType))]
+public partial class ComplexExpressionParser
+{
+    [Rule("program: expression ';'")]
+    public static int Program(int n, IToken _) => n;
+
+    [Rule("conditional_expression: primary_expression")]
+    [Rule("assignment_expression: conditional_expression")]
+    [Rule("constant_expression: conditional_expression")]
+    [Rule("expression: assignment_expression")]
+    [Rule("expression: constant_expression")]
+    public static int TrivialRules(int n) => n;
+
+    [Rule("conditional_expression: primary_expression '?' expression ':' conditional_expression")]
+    public static int MakeConditionalExpression(int condition, IToken _1, int trueExpression, IToken _2, int falseExpression) => condition == 0 ? falseExpression : trueExpression;
+
+    [Rule("assignment_expression: primary_expression '=' assignment_expression")]
+    public static int MakeConditionalExpression(int condition, IToken _1, int trueExpression) => trueExpression;
+
+    [Rule("primary_expression : '(' expression ')'")]
+    public static int Grouping(IToken _1, int n, IToken _2) => n;
+
+    [Rule("primary_expression : IntLit")]
+    public static int IntLit(IToken token) => int.Parse(token.Text);
+}
+
+[Parser(typeof(TokenType))]
+public partial class ManualComplexExpressionParser
+{
+    [Rule("program: expression ';'")]
+    public static int Program(int n, IToken _) => n;
+
+    /**
+     * This is optimized parse tree.
+     * expression - assignment_expression
+     *   
+     * assignment_expression -- primary_expression
+     *                     \--- primary_expression '?' expression ':' conditional_expression
+     *                     \--- primary_expression '=' assignment_expression
+     *
+     * conditional_expression - primary_expression
+     *                     \--- primary_expression '?' expression ':' conditional_expression
+     *            
+     */
+    [Rule("conditional_expression: primary_expression")]
+    [Rule("assignment_expression: primary_expression")]
+    [Rule("constant_expression: conditional_expression")]
+    [Rule("expression: assignment_expression")]
+    public static int TrivialRules(int n) => n;
+
+    [Rule("conditional_expression: primary_expression '?' expression ':' conditional_expression")]
+    [Rule("assignment_expression: primary_expression '?' expression ':' conditional_expression")]
+    public static int MakeConditionalExpression(int condition, IToken _1, int trueExpression, IToken _2, int falseExpression) => condition == 0 ? falseExpression : trueExpression;
+
+    [Rule("assignment_expression: primary_expression '=' assignment_expression")]
+    public static int MakeConditionalExpression(int condition, IToken _1, int trueExpression) => trueExpression;
+
+    [Rule("primary_expression : '(' expression ')'")]
+    public static int Grouping(IToken _1, int n, IToken _2) => n;
+
+    [Rule("primary_expression : IntLit")]
+    public static int IntLit(IToken token) => int.Parse(token.Text);
+}
+
diff --git a/Sources/SynKit/Benchmarks/Parser.Benchmarks/Program.cs b/Sources/SynKit/Benchmarks/Parser.Benchmarks/Program.cs
index d7d8cdda..d6fa8c44 100644
--- a/Sources/SynKit/Benchmarks/Parser.Benchmarks/Program.cs
+++ b/Sources/SynKit/Benchmarks/Parser.Benchmarks/Program.cs
@@ -16,6 +16,14 @@
 {
     new Yoakke.SynKit.Parser.Benchmarks.ExpressionBenchmarks().WorstManualExpressionParser();
 }
+else if (args.Length == 1 && args[0] == "complex")
+{
+    new Yoakke.SynKit.Parser.Benchmarks.ExpressionBenchmarks().ComplexExpressionParser();
+}
+else if (args.Length == 1 && args[0] == "manual_complex")
+{
+    new Yoakke.SynKit.Parser.Benchmarks.ExpressionBenchmarks().ManualComplexExpressionParser();
+}
 else
 {
    BenchmarkSwitcher.FromAssembly(typeof(Program).Assembly).Run(args);
diff --git a/Sources/SynKit/Benchmarks/Parser.Benchmarks/Properties/launchSettings.json b/Sources/SynKit/Benchmarks/Parser.Benchmarks/Properties/launchSettings.json
index bd7d20b1..3c72ae8d 100644
--- a/Sources/SynKit/Benchmarks/Parser.Benchmarks/Properties/launchSettings.json
+++ b/Sources/SynKit/Benchmarks/Parser.Benchmarks/Properties/launchSettings.json
@@ -3,9 +3,17 @@
     "Parser.Benchmarks": {
       "commandName": "Project"
     },
-    "Parser": {
+    "parser": {
       "commandName": "Project",
       "commandLineArgs": "parser"
+    },
+    "complex": {
+      "commandName": "Project",
+      "commandLineArgs": "complex"
+    },
+    "manual_complex": {
+      "commandName": "Project",
+      "commandLineArgs": "manual_complex"
     }
   }
-}
\ No newline at end of file
+}