diff --git a/.gitignore b/.gitignore
index a6dc78d..2e10a38 100644
--- a/.gitignore
+++ b/.gitignore
@@ -4,6 +4,7 @@
 /release/
 /.vscode/
 /vcpkg_installed/
+/_codeql_build_dir/
 
 CMakeUserPresets.json
 node_modules
diff --git a/generated/include/grammar/syntax.h b/generated/include/grammar/syntax.h
index 1d050e6..d96909b 100644
--- a/generated/include/grammar/syntax.h
+++ b/generated/include/grammar/syntax.h
@@ -29,20 +29,18 @@ constexpr alioth::SymbolID DOT = 11;
 constexpr alioth::SymbolID ELLIPSES = 12;
 constexpr alioth::SymbolID EMPTY = 13;
 constexpr alioth::SymbolID LANG = 14;
-constexpr alioth::SymbolID USING = 15;
-constexpr alioth::SymbolID STRING = 16;
-constexpr alioth::SymbolID ID = 17;
-constexpr alioth::SymbolID REGEX = 18;
-constexpr alioth::SymbolID COMMENT = 19;
-constexpr alioth::SymbolID SPACE = 20;
+constexpr alioth::SymbolID STRING = 15;
+constexpr alioth::SymbolID ID = 16;
+constexpr alioth::SymbolID REGEX = 17;
+constexpr alioth::SymbolID COMMENT = 18;
+constexpr alioth::SymbolID SPACE = 19;
 
-struct EmptyFormula; // SymbolID = 33; Accepts: [33]
-struct Formula; // SymbolID = 32; Accepts: [32, 34]
-struct Grammar; // SymbolID = 22; Accepts: [22]
-struct Ntrm; // SymbolID = 28; Accepts: [28]
-struct Symbol; // SymbolID = 35; Accepts: [35]
-struct Term; // SymbolID = 27; Accepts: [27]
-struct Using; // SymbolID = 26; Accepts: [26]
+struct EmptyFormula; // SymbolID = 30
+struct Formula; // SymbolID = 29
+struct Grammar; // SymbolID = 21
+struct Ntrm; // SymbolID = 25
+struct Symbol; // SymbolID = 32
+struct Term; // SymbolID = 24
 
 
 
@@ -61,7 +59,6 @@ struct Grammar : public alioth::ASTView {
   alioth::AST lang() const;
   std::vector<Ntrm> ntrms() const;
   std::vector<Term> terms() const;
-  std::vector<Using> usings() const;
   using ASTView::ASTView;
 };
 
@@ -69,7 +66,6 @@ struct Ntrm : public alioth::ASTView {
   struct Define;
   struct External;
   struct Formed;
-  struct Using;
   
   alioth::AST name() const;
   using ASTView::ASTView;
@@ -93,12 +89,6 @@ struct Ntrm::Formed: public Ntrm {
   std::vector<alioth::AST> formulas() const;
   alioth::AST name() const;
 };
-struct Ntrm::Using: public Ntrm {
-  using Ntrm::Ntrm;
-
-  alioth::AST lang() const;
-  alioth::AST name() const;
-};
 
 
 struct Symbol : public alioth::ASTView {
@@ -109,30 +99,10 @@ struct Symbol : public alioth::ASTView {
 };
 
 struct Term : public alioth::ASTView {
-  struct Define;
-  struct Using;
-  
-  alioth::AST name() const;
-  using ASTView::ASTView;
-};
-struct Term::Define: public Term {
-  using Term::Term;
-
   std::vector<alioth::AST> contexts() const;
   alioth::AST name() const;
   alioth::AST optional() const;
   alioth::AST regex() const;
-};
-struct Term::Using: public Term {
-  using Term::Term;
-
-  alioth::AST lang() const;
-  alioth::AST name() const;
-};
-
-
-struct Using : public alioth::ASTView {
-  alioth::AST grammar() const;
   using ASTView::ASTView;
 };
 
@@ -143,7 +113,7 @@ namespace alioth {
 template<>
 inline grammar::EmptyFormula alioth::ASTNode::As<grammar::EmptyFormula>() {
   switch(id) {
-    case 33:
+    case 30:
     return grammar::EmptyFormula{shared_from_this()};
   default:
     return {};
@@ -153,7 +123,7 @@ inline grammar::EmptyFormula alioth::ASTNode::As<grammar::EmptyFormula>() {
 template<>
 inline grammar::Formula alioth::ASTNode::As<grammar::Formula>() {
   switch(id) {
-    case 32:case 34:
+    case 29: case 31:
     return grammar::Formula{shared_from_this()};
   default:
     return {};
@@ -163,7 +133,7 @@ inline grammar::Formula alioth::ASTNode::As<grammar::Formula>() {
 template<>
 inline grammar::Grammar alioth::ASTNode::As<grammar::Grammar>() {
   switch(id) {
-    case 22:
+    case 21:
     return grammar::Grammar{shared_from_this()};
   default:
     return {};
@@ -173,7 +143,7 @@ inline grammar::Grammar alioth::ASTNode::As<grammar::Grammar>() {
 template<>
 inline grammar::Ntrm alioth::ASTNode::As<grammar::Ntrm>() {
   switch(id) {
-    case 28:
+    case 25:
     return grammar::Ntrm{shared_from_this()};
   default:
     return {};
@@ -182,7 +152,7 @@ inline grammar::Ntrm alioth::ASTNode::As<grammar::Ntrm>() {
 template<>
 inline grammar::Ntrm::Define alioth::ASTNode::As<grammar::Ntrm::Define>() {
   switch(OriginFormula()) {
-    case 24:
+    case 16:
     return grammar::Ntrm::Define{shared_from_this()};
   default:
     return {};
@@ -191,7 +161,7 @@ inline grammar::Ntrm::Define alioth::ASTNode::As<grammar::Ntrm::Define>() {
 template<>
 inline grammar::Ntrm::External alioth::ASTNode::As<grammar::Ntrm::External>() {
   switch(OriginFormula()) {
-    case 26:case 27:
+    case 18:
     return grammar::Ntrm::External{shared_from_this()};
   default:
     return {};
@@ -200,27 +170,18 @@ inline grammar::Ntrm::External alioth::ASTNode::As<grammar::Ntrm::External>() {
 template<>
 inline grammar::Ntrm::Formed alioth::ASTNode::As<grammar::Ntrm::Formed>() {
   switch(OriginFormula()) {
-    case 25:
+    case 17:
     return grammar::Ntrm::Formed{shared_from_this()};
   default:
     return {};
   }
 }
-template<>
-inline grammar::Ntrm::Using alioth::ASTNode::As<grammar::Ntrm::Using>() {
-  switch(OriginFormula()) {
-    case 28:
-    return grammar::Ntrm::Using{shared_from_this()};
-  default:
-    return {};
-  }
-}
 
 
 template<>
 inline grammar::Symbol alioth::ASTNode::As<grammar::Symbol>() {
   switch(id) {
-    case 35:
+    case 32:
     return grammar::Symbol{shared_from_this()};
   default:
     return {};
@@ -230,41 +191,12 @@ inline grammar::Symbol alioth::ASTNode::As<grammar::Symbol>() {
 template<>
 inline grammar::Term alioth::ASTNode::As<grammar::Term>() {
   switch(id) {
-    case 27:
+    case 24:
     return grammar::Term{shared_from_this()};
   default:
     return {};
   }
 }
-template<>
-inline grammar::Term::Define alioth::ASTNode::As<grammar::Term::Define>() {
-  switch(OriginFormula()) {
-    case 16:case 17:case 18:case 19:
-    return grammar::Term::Define{shared_from_this()};
-  default:
-    return {};
-  }
-}
-template<>
-inline grammar::Term::Using alioth::ASTNode::As<grammar::Term::Using>() {
-  switch(OriginFormula()) {
-    case 20:
-    return grammar::Term::Using{shared_from_this()};
-  default:
-    return {};
-  }
-}
-
-
-template<>
-inline grammar::Using alioth::ASTNode::As<grammar::Using>() {
-  switch(id) {
-    case 26:
-    return grammar::Using{shared_from_this()};
-  default:
-    return {};
-  }
-}
 
 
 template<>
@@ -273,87 +205,83 @@ inline Syntax SyntaxOf<grammar::Grammar>() {
     using namespace nlohmann;
     auto lex = Lexicon::Builder("grammar");
     lex.Define("ARROW", R"(->)"_regex);
-    
     lex.Define("LT", R"(<)"_regex);
-    
     lex.Define("GT", R"(>)"_regex);
-    
     lex.Define("OR", R"(\|)"_regex);
-    
     lex.Define("ASSIGN", R"(=)"_regex);
-    
     lex.Define("OPTIONAL", R"(\?)"_regex);
-    
     lex.Define("AT", R"(@)"_regex);
-    
     lex.Define("SEMICOLON", R"(;)"_regex);
-    
     lex.Define("COLON", R"(:)"_regex);
-    
     lex.Define("COMMA", R"(,)"_regex);
-    
     lex.Define("DOT", R"(\.)"_regex);
-    
     lex.Define("ELLIPSES", R"(\.\.\.)"_regex);
-    
     lex.Define("EMPTY", R"(%empty)"_regex);
-    
     lex.Define("LANG", R"(lang)"_regex, { "keyword",  });
-    
-    lex.Define("USING", R"(using)"_regex, { "keyword",  });
-    
     lex.Define("STRING", R"(\"([^\"\n\\]|\\[^\n])*\")"_regex, { "json",  });
-    
     lex.Define("ID", R"([a-zA-Z_]\w*)"_regex);
-    
     lex.Define("REGEX", R"(\/([^\\\/]|\\[^\n])+\/)"_regex);
-    
     lex.Define("COMMENT", R"(#[^\n]*\n)"_regex);
-    
     lex.Define("SPACE", R"(\s+)"_regex);
     
-    
     auto syntax = Syntactic::Builder(lex.Build());
     syntax.Ignore("SEMICOLON");
     syntax.Ignore("COMMENT");
     syntax.Ignore("SPACE");
     
+    // grammar -> LANG COLON ID@lang ...terms? ...ntrms?;
     syntax.Formula("grammar").Symbol("LANG").Symbol("COLON").Symbol("ID", "lang").Commit();
-    syntax.Formula("grammar").Symbol("LANG").Symbol("COLON").Symbol("ID", "lang").Symbol("usings", "...").Commit();
     syntax.Formula("grammar").Symbol("LANG").Symbol("COLON").Symbol("ID", "lang").Symbol("terms", "...").Commit();
-    syntax.Formula("grammar").Symbol("LANG").Symbol("COLON").Symbol("ID", "lang").Symbol("usings", "...").Symbol("terms", "...").Commit();
     syntax.Formula("grammar").Symbol("LANG").Symbol("COLON").Symbol("ID", "lang").Symbol("ntrms", "...").Commit();
-    syntax.Formula("grammar").Symbol("LANG").Symbol("COLON").Symbol("ID", "lang").Symbol("usings", "...").Symbol("ntrms", "...").Commit();
     syntax.Formula("grammar").Symbol("LANG").Symbol("COLON").Symbol("ID", "lang").Symbol("terms", "...").Symbol("ntrms", "...").Commit();
-    syntax.Formula("grammar").Symbol("LANG").Symbol("COLON").Symbol("ID", "lang").Symbol("usings", "...").Symbol("terms", "...").Symbol("ntrms", "...").Commit();
-    syntax.Formula("usings").Symbol("using", "usings").Commit();
-    syntax.Formula("usings").Symbol("usings", "...").Symbol("using", "usings").Commit();
+    
+    // terms -> ...terms? term@terms;
     syntax.Formula("terms").Symbol("term", "terms").Commit();
     syntax.Formula("terms").Symbol("terms", "...").Symbol("term", "terms").Commit();
+    
+    // ntrms -> ...ntrms? ntrm@ntrms;
     syntax.Formula("ntrms").Symbol("ntrm", "ntrms").Commit();
     syntax.Formula("ntrms").Symbol("ntrms", "...").Symbol("ntrm", "ntrms").Commit();
-    syntax.Formula("using").Symbol("USING").Symbol("STRING", "grammar").Commit();
-    syntax.Formula("term", "define").Symbol("ID", "name").Symbol("ASSIGN").Symbol("REGEX", "regex").Commit();
-    syntax.Formula("term", "define").Symbol("ID", "name").Symbol("contexts", "...").Symbol("ASSIGN").Symbol("REGEX", "regex").Commit();
-    syntax.Formula("term", "define").Symbol("ID", "name").Symbol("OPTIONAL", "optional").Symbol("ASSIGN").Symbol("REGEX", "regex").Commit();
-    syntax.Formula("term", "define").Symbol("ID", "name").Symbol("contexts", "...").Symbol("OPTIONAL", "optional").Symbol("ASSIGN").Symbol("REGEX", "regex").Commit();
-    syntax.Formula("term", "using").Symbol("ELLIPSES").Symbol("ASSIGN").Symbol("ID", "lang").Symbol("DOT").Symbol("ID", "name").Commit();
+    
+    // term -> ID@name ...contexts? OPTIONAL?@optional ASSIGN REGEX@regex;
+    syntax.Formula("term").Symbol("ID", "name").Symbol("ASSIGN").Symbol("REGEX", "regex").Commit();
+    syntax.Formula("term").Symbol("ID", "name").Symbol("contexts", "...").Symbol("ASSIGN").Symbol("REGEX", "regex").Commit();
+    syntax.Formula("term").Symbol("ID", "name").Symbol("OPTIONAL", "optional").Symbol("ASSIGN").Symbol("REGEX", "regex").Commit();
+    syntax.Formula("term").Symbol("ID", "name").Symbol("contexts", "...").Symbol("OPTIONAL", "optional").Symbol("ASSIGN").Symbol("REGEX", "regex").Commit();
+    
+    // contexts -> LT ...context_list GT;
     syntax.Formula("contexts").Symbol("LT").Symbol("context_list", "...").Symbol("GT").Commit();
+    
+    // context_list -> ID@contexts | ...context_list COMMA ID@contexts;
     syntax.Formula("context_list").Symbol("ID", "contexts").Commit();
     syntax.Formula("context_list").Symbol("context_list", "...").Symbol("COMMA").Symbol("ID", "contexts").Commit();
+    
+    // ntrm.define -> ID@name ARROW ...formula_group SEMICOLON;
     syntax.Formula("ntrm", "define").Symbol("ID", "name").Symbol("ARROW").Symbol("formula_group", "...").Symbol("SEMICOLON").Commit();
+    
+    // ntrm.formed -> ID@name DOT ID@form ARROW ...formula_group SEMICOLON;
     syntax.Formula("ntrm", "formed").Symbol("ID", "name").Symbol("DOT").Symbol("ID", "form").Symbol("ARROW").Symbol("formula_group", "...").Symbol("SEMICOLON").Commit();
+    
+    // ntrm.external -> ID@name ARROW STRING@grammar SEMICOLON;
     syntax.Formula("ntrm", "external").Symbol("ID", "name").Symbol("ARROW").Symbol("STRING", "grammar").Symbol("SEMICOLON").Commit();
-    syntax.Formula("ntrm", "external").Symbol("ELLIPSES").Symbol("ARROW").Symbol("STRING", "grammar").Symbol("SEMICOLON").Commit();
-    syntax.Formula("ntrm", "using").Symbol("ELLIPSES").Symbol("ARROW").Symbol("ID", "lang").Symbol("DOT").Symbol("ID", "name").Symbol("SEMICOLON").Commit();
+    
+    // formula_group -> formula@formulas | empty_formula@formulas | ...formula_group OR formula@formulas | ...formula_group OR empty_formula@formulas;
     syntax.Formula("formula_group").Symbol("formula", "formulas").Commit();
     syntax.Formula("formula_group").Symbol("empty_formula", "formulas").Commit();
     syntax.Formula("formula_group").Symbol("formula_group", "...").Symbol("OR").Symbol("formula", "formulas").Commit();
     syntax.Formula("formula_group").Symbol("formula_group", "...").Symbol("OR").Symbol("empty_formula", "formulas").Commit();
+    
+    // formula -> ...formula_body;
     syntax.Formula("formula").Symbol("formula_body", "...").Commit();
+    
+    // formula_body -> ...formula_body? symbol@symbols;
     syntax.Formula("formula_body").Symbol("symbol", "symbols").Commit();
     syntax.Formula("formula_body").Symbol("formula_body", "...").Symbol("symbol", "symbols").Commit();
+    
+    // empty_formula -> EMPTY@empty;
     syntax.Formula("empty_formula").Symbol("EMPTY", "empty").Commit();
+    
+    // symbol -> ID@name OPTIONAL?@optional | ID@name OPTIONAL?@optional AT ID@attr | ELLIPSES@attr ID@name OPTIONAL?@optional;
     syntax.Formula("symbol").Symbol("ID", "name").Commit();
     syntax.Formula("symbol").Symbol("ID", "name").Symbol("OPTIONAL", "optional").Commit();
     syntax.Formula("symbol").Symbol("ID", "name").Symbol("AT").Symbol("ID", "attr").Commit();
@@ -400,14 +328,6 @@ inline std::vector<Term> Grammar::terms() const {
     }
   );
 }
-inline std::vector<Using> Grammar::usings() const { 
-  return alioth::generic::collect<alioth::generic::multiple>(
-    (*this)->Attrs("usings"), 
-    [](auto n) {
-      return n->template As<Using>();
-    }
-  );
-}
 inline std::vector<alioth::AST> Ntrm::Define::formulas() const { 
   return (*this)->Attrs("formulas"); 
 }
@@ -429,12 +349,6 @@ inline std::vector<alioth::AST> Ntrm::Formed::formulas() const {
 inline alioth::AST Ntrm::Formed::name() const { 
   return (*this)->Attr("name");
 }
-inline alioth::AST Ntrm::Using::lang() const { 
-  return (*this)->Attr("lang");
-}
-inline alioth::AST Ntrm::Using::name() const { 
-  return (*this)->Attr("name");
-}
 
 inline alioth::AST Ntrm::name() const { 
   return (*this)->Attr("name");
@@ -448,33 +362,20 @@ inline alioth::AST Symbol::name() const {
 inline alioth::AST Symbol::optional() const { 
   return (*this)->Attr("optional");
 }
-inline std::vector<alioth::AST> Term::Define::contexts() const { 
+inline std::vector<alioth::AST> Term::contexts() const { 
   return (*this)->Attrs("contexts"); 
 }
-inline alioth::AST Term::Define::name() const { 
+inline alioth::AST Term::name() const { 
   return (*this)->Attr("name");
 }
-inline alioth::AST Term::Define::optional() const { 
+inline alioth::AST Term::optional() const { 
   return (*this)->Attr("optional");
 }
-inline alioth::AST Term::Define::regex() const { 
+inline alioth::AST Term::regex() const { 
   return (*this)->Attr("regex");
 }
-inline alioth::AST Term::Using::lang() const { 
-  return (*this)->Attr("lang");
-}
-inline alioth::AST Term::Using::name() const { 
-  return (*this)->Attr("name");
-}
-
-inline alioth::AST Term::name() const { 
-  return (*this)->Attr("name");
-}
-inline alioth::AST Using::grammar() const { 
-  return (*this)->Attr("grammar");
-}
 
 
 }
 
-#endif
\ No newline at end of file
+#endif
diff --git a/grammar/grammar.grammar b/grammar/grammar.grammar
index c14867a..54e57c6 100644
--- a/grammar/grammar.grammar
+++ b/grammar/grammar.grammar
@@ -17,31 +17,24 @@ DOT = /\./
 ELLIPSES = /\.\.\./
 EMPTY = /%empty/
 LANG<keyword> = /lang/
-USING<keyword> = /using/
 STRING<json> = /\"([^\"\n\\]|\\[^\n])*\"/
 ID = /[a-zA-Z_]\w*/
 REGEX = /\/([^\\\/]|\\[^\n])+\//
 COMMENT ?= /#[^\n]*\n/
 SPACE ?= /\s+/
 
-grammar -> LANG COLON ID@lang ...usings? ...terms? ...ntrms?;
-usings -> ...usings? using@usings;
+grammar -> LANG COLON ID@lang ...terms? ...ntrms?;
 terms -> ...terms? term@terms;
 ntrms -> ...ntrms? ntrm@ntrms;
 
-using -> USING STRING@grammar;
-
-term.define -> ID@name ...contexts? OPTIONAL?@optional ASSIGN REGEX@regex;
-term.using -> ELLIPSES ASSIGN ID@lang DOT ID@name;
+term -> ID@name ...contexts? OPTIONAL?@optional ASSIGN REGEX@regex;
 contexts -> LT ...context_list GT;
 context_list -> ID@contexts
     | ...context_list COMMA ID@contexts;
 
-ntrm.define -> ID@name  ARROW ...formula_group SEMICOLON;
+ntrm.define -> ID@name ARROW ...formula_group SEMICOLON;
 ntrm.formed -> ID@name DOT ID@form ARROW ...formula_group SEMICOLON;
 ntrm.external -> ID@name ARROW STRING@grammar SEMICOLON;
-ntrm.external -> ELLIPSES ARROW STRING@grammar SEMICOLON;
-ntrm.using -> ELLIPSES ARROW ID@lang DOT ID@name SEMICOLON;
 
 formula_group -> formula@formulas
     | empty_formula@formulas
diff --git a/include/alioth/println_compat.h b/include/alioth/println_compat.h
new file mode 100644
index 0000000..61f6052
--- /dev/null
+++ b/include/alioth/println_compat.h
@@ -0,0 +1,25 @@
+#ifndef __ALIOTH_PRINTLN_COMPAT_H__
+#define __ALIOTH_PRINTLN_COMPAT_H__
+
+#include <fmt/core.h>
+#include <cstdio>
+
+namespace fmt {
+// fmt::println was added in fmt 10.0.0 (FMT_VERSION >= 100000)
+// This provides a compatibility implementation for older versions
+#if FMT_VERSION < 100000
+template <typename S, typename... Args>
+inline void println(FILE* f, const S& format, Args&&... args) {
+  print(f, fmt::runtime(format), std::forward<Args>(args)...);
+  std::fputc('\n', f);
+}
+
+template <typename S, typename... Args>
+inline void println(const S& format, Args&&... args) {
+  print(fmt::runtime(format), std::forward<Args>(args)...);
+  std::putchar('\n');
+}
+#endif
+}
+
+#endif
diff --git a/include/aliox/grammar.h b/include/aliox/grammar.h
index 54d3d52..24d9012 100644
--- a/include/aliox/grammar.h
+++ b/include/aliox/grammar.h
@@ -1,6 +1,8 @@
 #ifndef __ALIOX_GRAMMAR_H__
 #define __ALIOX_GRAMMAR_H__
 
+#include <functional>
+#include <string>
 #include <vector>
 
 #include "alioth/ast.h"
@@ -8,6 +10,15 @@
 
 namespace alioth {
 
+/**
+ * 外部语法加载器类型
+ * 
+ * 用于加载外部符号引用的语法规则
+ * 参数为外部语法的路径或标识符
+ * 返回对应的语法规则，如果无法加载则返回nullptr
+ */
+using ExternalLoader = std::function<Syntax(std::string const&)>;
+
 struct Grammar {
   /**
    * 将文法定义编译为语法规则
@@ -15,6 +26,14 @@ struct Grammar {
    * @param grammar 文法源码
    */
   static Syntax Compile(Doc grammar);
+
+  /**
+   * 将文法定义编译为语法规则
+   *
+   * @param grammar 文法源码
+   * @param loader 外部语法加载器，用于加载外部符号引用的语法
+   */
+  static Syntax Compile(Doc grammar, ExternalLoader loader);
 };
 
 }  // namespace alioth
diff --git a/src/alioth-cli/parse.cpp b/src/alioth-cli/parse.cpp
index c5b1cb0..d5b6e1b 100644
--- a/src/alioth-cli/parse.cpp
+++ b/src/alioth-cli/parse.cpp
@@ -3,6 +3,7 @@
 #include "alioth-cli/syntax.h"
 #include "alioth/document.h"
 #include "alioth/parser.h"
+#include "alioth/println_compat.h"
 #include "aliox/grammar.h"
 #include "aliox/skeleton.h"
 
diff --git a/src/alioth-cli/skeleton.cpp b/src/alioth-cli/skeleton.cpp
index a94c75d..e017c35 100644
--- a/src/alioth-cli/skeleton.cpp
+++ b/src/alioth-cli/skeleton.cpp
@@ -4,6 +4,7 @@
 #include "alioth/alioth.h"
 #include "alioth/document.h"
 #include "alioth/parser.h"
+#include "alioth/println_compat.h"
 #include "alioth/strings.h"
 #include "aliox/grammar.h"
 #include "aliox/skeleton.h"
diff --git a/src/alioth/parser.cpp b/src/alioth/parser.cpp
index f7752ac..ef6a9b4 100644
--- a/src/alioth/parser.cpp
+++ b/src/alioth/parser.cpp
@@ -1,5 +1,7 @@
 #include "alioth/parser.h"
 
+#include "alioth/println_compat.h"
+
 namespace alioth {
 
 Parser::Parser(Syntax syntax, Doc doc) : Parser(syntax, doc, {}) {}
diff --git a/src/alioth/syntax.cpp b/src/alioth/syntax.cpp
index a27d816..cab3a62 100644
--- a/src/alioth/syntax.cpp
+++ b/src/alioth/syntax.cpp
@@ -2,6 +2,8 @@
 
 #include <tuple>
 
+#include "alioth/println_compat.h"
+
 namespace alioth {
 
 bool Syntactic::IsTerm(SymbolID id) const {
diff --git a/src/aliox/grammar.cpp b/src/aliox/grammar.cpp
index 6f2a016..37f883b 100644
--- a/src/aliox/grammar.cpp
+++ b/src/aliox/grammar.cpp
@@ -40,6 +40,10 @@ std::optional<std::string> GetForm(grammar::Ntrm const& ntrm) {
 }  // namespace
 
 Syntax Grammar::Compile(Doc grammar) {
+  return Compile(grammar, {});
+}
+
+Syntax Grammar::Compile(Doc grammar, ExternalLoader loader) {
   auto root = Parser(SyntaxOf<grammar::Grammar>(), grammar).Parse();
   auto g = ViewOf<grammar::Grammar>(root);
 
@@ -49,48 +53,36 @@ Syntax Grammar::Compile(Doc grammar) {
   
   // 处理终结符定义
   for (auto const& term : g.terms()) {
-    // 跳过 term.using，它是复用其他语言的终结符
-    // 在简化版中，我们不支持这个功能
-    auto use = term->As<grammar::Term::Using>();
-    if (use) {
-      continue;
-    }
-    
-    auto define = term->As<grammar::Term::Define>();
-    if (!define) continue;
-    
-    auto src = define.regex()->Text();
+    auto src = term.regex()->Text();
     auto regex = RegexTree::Compile(src.substr(1, src.size() - 2));
 
-    lex.Define(define.name()->Text(), regex, collect(define.contexts(), text()));
+    lex.Define(term.name()->Text(), regex, collect(term.contexts(), text()));
   }
 
   auto syntax = Syntactic::Builder(lex.Build());
 
   // 标记可忽略的终结符
   for (auto const& term : g.terms()) {
-    auto define = term->As<grammar::Term::Define>();
-    if (!define) continue;
-    
-    if (define.optional()) {
-      syntax.Ignore(define.name()->Text());
+    if (term.optional()) {
+      syntax.Ignore(term.name()->Text());
     }
   }
 
   // 处理非终结符定义
   for (auto const& ntrm : g.ntrms()) {
-    // 跳过 ntrm.external，它是引用外部文法
-    // 保留对外部符号机制的支持，但在这个简化版中只是跳过
+    // 处理外部符号: 导入其他语言作为外部符号
     auto external = ntrm->As<grammar::Ntrm::External>();
     if (external) {
-      // TODO: 外部非终结符需要从其他语言加载
-      continue;
-    }
-    
-    // 跳过 ntrm.using，它是复用其他语言的非终结符
-    // 在简化版中，我们不支持这个功能
-    auto use = ntrm->As<grammar::Ntrm::Using>();
-    if (use) {
+      auto grammarPath = external.grammar()->Text();
+      // 去除引号
+      grammarPath = grammarPath.substr(1, grammarPath.size() - 2);
+      
+      if (loader) {
+        auto externalSyntax = loader(grammarPath);
+        if (externalSyntax) {
+          syntax.Import(externalSyntax, external.name()->Text());
+        }
+      }
       continue;
     }
     
diff --git a/src/aliox/template.cpp b/src/aliox/template.cpp
index d306eda..63992ac 100644
--- a/src/aliox/template.cpp
+++ b/src/aliox/template.cpp
@@ -4,6 +4,7 @@
 
 #include "alioth/alioth.h"
 #include "alioth/parser.h"
+#include "alioth/println_compat.h"
 #include "aliox/grammar.h"
 #include "fmt/format.h"
 #include "nlohmann/json.hpp"