diff --git a/CMakeLists.txt b/CMakeLists.txt
index 073f507..e0abd24 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -20,5 +20,8 @@ target_link_libraries(calc2 pthread)
 add_executable(calc3 example/calc3.cc)
 target_link_libraries(calc3 pthread)
 
+add_executable(pl0 example/pl0.cc)
+target_link_libraries(pl0 pthread)
+
 add_executable(culebra language/main.cc)
 target_link_libraries(culebra pthread)
diff --git a/example/Makefile b/example/Makefile
index 10237a9..9daa22e 100644
--- a/example/Makefile
+++ b/example/Makefile
@@ -19,3 +19,6 @@ calc2 : calc2.cc ../peglib.h
 
 calc3 : calc3.cc ../peglib.h
 	$(CC) -o calc3 $(CFLAGS) -I.. calc3.cc
+
+pl0 : pl0.cc ../peglib.h
+	$(CC) -o pl0 $(CFLAGS) -I.. pl0.cc
diff --git a/example/example.sln b/example/example.sln
index f88ef2b..4b46397 100644
--- a/example/example.sln
+++ b/example/example.sln
@@ -1,7 +1,7 @@
 
 Microsoft Visual Studio Solution File, Format Version 12.00
-# Visual Studio 2013
-VisualStudioVersion = 12.0.31101.0
+# Visual Studio 14
+VisualStudioVersion = 14.0.23107.0
 MinimumVisualStudioVersion = 10.0.40219.1
 Project("{8BC9CEB8-8B4A-11D0-8D11-00A0C91BC942}") = "calc", "calc.vcxproj", "{F85B641A-7538-4809-8175-C528FF632CF6}"
 EndProject
@@ -9,6 +9,8 @@ Project("{8BC9CEB8-8B4A-11D0-8D11-00A0C91BC942}") = "calc2", "calc2.vcxproj", "{
 EndProject
 Project("{8BC9CEB8-8B4A-11D0-8D11-00A0C91BC942}") = "calc3", "calc3.vcxproj", "{E6146F73-3B4C-4D4C-BC55-148930954434}"
 EndProject
+Project("{8BC9CEB8-8B4A-11D0-8D11-00A0C91BC942}") = "pl0", "pl0.vcxproj", "{6C5633BD-3CAE-498E-B0C6-ED90A1A99C47}"
+EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		Debug|Win32 = Debug|Win32
@@ -27,6 +29,10 @@ Global
 		{E6146F73-3B4C-4D4C-BC55-148930954434}.Debug|Win32.Build.0 = Debug|Win32
 		{E6146F73-3B4C-4D4C-BC55-148930954434}.Release|Win32.ActiveCfg = Release|Win32
 		{E6146F73-3B4C-4D4C-BC55-148930954434}.Release|Win32.Build.0 = Release|Win32
+		{6C5633BD-3CAE-498E-B0C6-ED90A1A99C47}.Debug|Win32.ActiveCfg = Debug|Win32
+		{6C5633BD-3CAE-498E-B0C6-ED90A1A99C47}.Debug|Win32.Build.0 = Debug|Win32
+		{6C5633BD-3CAE-498E-B0C6-ED90A1A99C47}.Release|Win32.ActiveCfg = Release|Win32
+		{6C5633BD-3CAE-498E-B0C6-ED90A1A99C47}.Release|Win32.Build.0 = Release|Win32
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE
diff --git a/example/pl0.cc b/example/pl0.cc
new file mode 100644
index 0000000..e94ae21
--- /dev/null
+++ b/example/pl0.cc
@@ -0,0 +1,353 @@
+//
+//  pl0.cc - PL/0 interpreter (https://en.wikipedia.org/wiki/PL/0)
+//
+//  Copyright (c) 2015 Yuji Hirose. All rights reserved.
+//  MIT License
+//
+
+#include <peglib.h>
+#include <fstream>
+#include <iostream>
+
+using namespace peglib;
+using namespace std;
+
+auto grammar = R"(
+    program    <- _ block '.' _
+
+    block      <- const var procedure statement
+    const      <- ('CONST' _ ident '=' _ number (',' _ ident '=' _ number)* ';' _)?
+    var        <- ('VAR' _ ident (',' _ ident)* ';' _)?
+    procedure  <- ('PROCEDURE' _ ident ';' _ block ';' _)*
+
+    statement  <- (assignment / call / statements / if / while / out / in)?
+    assignment <- ident ':=' _ expression
+    call       <- 'CALL' _ ident
+    statements <- 'BEGIN' _ statement (';' _ statement )* 'END' _
+    if         <- 'IF' _ condition 'THEN' _ statement
+    while      <- 'WHILE' _ condition 'DO' _ statement
+    out        <- ('out' / 'write' / '!') _ expression
+    in         <- ('in' / 'read' / '?') _ ident
+
+    condition  <- odd / compare
+    odd        <- 'ODD' _ expression
+    compare    <- expression compare_op expression
+    compare_op <- < '=' / '#' / '<=' / '<' / '>=' / '>' > _
+
+    expression <- sign term (term_op term)*
+    sign       <- < [-+]? > _
+    term_op    <- < [-+] > _
+
+    term       <- factor (factor_op factor)*
+    factor_op  <- < [*/] > _
+
+    factor     <- ident / number / '(' _ expression ')' _
+
+    ident      <- < [a-z] ([a-z] / [0-9])* > _
+    number     <- < [0-9]+ > _
+
+    ~_         <- [ \t\r\n]*
+)";
+
+struct Environment
+{
+    Environment(shared_ptr<Environment> outer = nullptr) : outer(outer) {}
+
+    int get_value(const string& ident) const {
+        try {
+            return get_constant(ident);
+        } catch (...) {
+            return get_variable(ident);
+        }
+    }
+
+    void set_variable(const string& ident, int val) {
+        if (variables.find(ident) != variables.end()) {
+            variables[ident] = val;
+        } else if (outer) {
+            return outer->set_variable(ident, val);
+        } else {
+            throw runtime_error("undefined variable");
+        }
+    }
+
+    int get_constant(const string& ident) const {
+        if (constants.find(ident) != constants.end()) {
+            return constants.at(ident);
+        } else if (outer) {
+            return outer->get_constant(ident);
+        } else {
+            throw runtime_error("undefined constants");
+        }
+    }
+
+    int get_variable(const string& ident) const {
+        if (variables.find(ident) != variables.end()) {
+            return variables.at(ident);
+        } else if (outer) {
+            return outer->get_variable(ident);
+        } else {
+            throw runtime_error("undefined variable");
+        }
+    }
+
+    map<string, int>             constants;
+    map<string, int>             variables;
+    map<string, shared_ptr<Ast>> procedures;
+    shared_ptr<Environment>      outer;
+};
+
+struct Interpreter
+{
+    void exec(const shared_ptr<Ast> ast, shared_ptr<Environment> env) {
+        switch (ast->tag) {
+            case "block"_:      exec_block(ast, env); break;
+            case "statement"_:  exec_statement(ast, env); break;
+            case "assignment"_: exec_assignment(ast, env); break;
+            case "call"_:       exec_call(ast, env); break;
+            case "statements"_: exec_statements(ast, env); break;
+            case "if"_:         exec_if(ast, env); break;
+            case "while"_:      exec_while(ast, env); break;
+            case "out"_:        exec_out(ast, env); break;
+            case "in"_:         exec_in(ast, env); break;
+            default:            throw logic_error("invalid Ast type");
+        }
+    }
+
+private:
+    void exec_block(const shared_ptr<Ast> ast, shared_ptr<Environment> outer) {
+        // block <- const var procedure statement
+        auto env = make_shared<Environment>(outer);
+        const auto& nodes = ast->nodes;
+        exec_constants(nodes[0], env);
+        exec_variables(nodes[1], env);
+        exec_procedures(nodes[2], env);
+        exec(nodes[3], env);
+    }
+
+    void exec_constants(const shared_ptr<Ast> ast, shared_ptr<Environment> env) {
+        // const <- ('CONST' _ ident '=' _ number(',' _ ident '=' _ number)* ';' _) ?
+        const auto& nodes = ast->nodes;
+        for (auto i = 0u; i < nodes.size(); i += 2) {
+            const auto& ident = nodes[i + 0]->token;
+            auto number = stoi(nodes[i + 1]->token);
+            env->constants[ident] = number;
+        }
+    }
+
+    void exec_variables(const shared_ptr<Ast> ast, shared_ptr<Environment> env) {
+        // var <- ('VAR' _ ident(',' _ ident)* ';' _) ?
+        const auto& nodes = ast->nodes;
+        for (auto i = 0u; i < nodes.size(); i += 1) {
+            const auto& ident = nodes[i]->token;
+            env->variables[ident] = 0;
+        }
+    }
+
+    void exec_procedures(const shared_ptr<Ast> ast, shared_ptr<Environment> env) {
+        // procedure <- ('PROCEDURE' _ ident ';' _ block ';' _)*
+        const auto& nodes = ast->nodes;
+        for (auto i = 0u; i < nodes.size(); i += 2) {
+            const auto& ident = nodes[i + 0]->token;
+            auto block = nodes[i + 1];
+            env->procedures[ident] = block;
+        }
+    }
+
+    void exec_statement(const shared_ptr<Ast> ast, shared_ptr<Environment> env) {
+        // statement  <-(assignment / call / statements / if / while / out / in) ?
+        if (!ast->nodes.empty()) {
+            exec(ast->nodes[0], env);
+        }
+    }
+
+    void exec_assignment(const shared_ptr<Ast> ast, shared_ptr<Environment> env) {
+        // assignment <- ident ':=' _ expression
+        const auto& ident = ast->nodes[0]->token;
+        auto val = eval(ast->nodes[1], env);
+        env->set_variable(ident, val);
+    }
+
+    void exec_call(const shared_ptr<Ast> ast, shared_ptr<Environment> env) {
+        // call <- 'CALL' _ ident
+        const auto& ident = ast->nodes[0]->token;
+        auto proc = env->procedures[ident];
+        exec_block(proc, env);
+    }
+
+    void exec_statements(const shared_ptr<Ast> ast, shared_ptr<Environment> env) {
+        // statements <- 'BEGIN' _ statement (';' _ statement )* 'END' _
+        for (auto stmt: ast->nodes) {
+            exec(stmt, env);
+        }
+    }
+
+    void exec_if(const shared_ptr<Ast> ast, shared_ptr<Environment> env) {
+        // if <- 'IF' _ condition 'THEN' _ statement
+        auto cond = eval_condition(ast->nodes[0], env);
+        auto stmt = ast->nodes[1];
+        if (cond) {
+            exec(stmt, env);
+        }
+    }
+
+    void exec_while(const shared_ptr<Ast> ast, shared_ptr<Environment> env) {
+        // while <- 'WHILE' _ condition 'DO' _ statement
+        auto cond = ast->nodes[0];
+        auto stmt = ast->nodes[1];
+        auto ret = eval_condition(cond, env);
+        while (ret) {
+            exec(stmt, env);
+            ret = eval_condition(cond, env);
+        }
+    }
+
+    void exec_out(const shared_ptr<Ast> ast, shared_ptr<Environment> env) {
+        // out <- '!' _ expression
+        auto val = eval(ast->nodes[0], env);
+        cout << val << endl;
+    }
+
+    void exec_in(const shared_ptr<Ast> ast, shared_ptr<Environment> env) {
+        // in <- '?' _ ident
+        int val;
+        cin >> val;
+        const auto& ident = ast->nodes[0]->token;
+        env->variables[ident] = val;
+    }
+
+    bool eval_condition(const shared_ptr<Ast> ast, shared_ptr<Environment> env) {
+        // condition <- odd / compare
+        const auto& node = ast->nodes[0];
+        switch (node->tag) {
+            case "odd"_:     return eval_odd(node, env);
+            case "compare"_: return eval_compare(node, env);
+            default:         throw logic_error("invalid Ast type");
+        }
+    }
+
+    bool eval_odd(const shared_ptr<Ast> ast, shared_ptr<Environment> env) {
+        // odd <- 'ODD' _ expression
+        auto val = eval_expression(ast->nodes[0], env);
+        return val != 0;
+    }
+
+    bool eval_compare(const shared_ptr<Ast> ast, shared_ptr<Environment> env) {
+        // compare <- expression compare_op expression
+        auto lval = eval_expression(ast->nodes[0], env);
+        auto op = peglib::str2tag(ast->nodes[1]->token.c_str());
+        auto rval = eval_expression(ast->nodes[2], env);
+        switch (op) {
+            case "="_:  return lval == rval;
+            case "#"_:  return lval != rval;
+            case "<="_: return lval <= rval;
+            case "<"_:  return lval < rval;
+            case ">="_: return lval >= rval;
+            case ">"_:  return lval > rval;
+        }
+    }
+
+    int eval(const shared_ptr<Ast> ast, shared_ptr<Environment> env) {
+        switch (ast->tag) {
+            case "expression"_: return eval_expression(ast, env);
+            case "term"_:       return eval_term(ast, env);
+            case "ident"_:      return eval_ident(ast, env);
+            case "number"_:     return eval_number(ast, env);
+            default:            throw logic_error("invalid Ast type");
+        }
+    }
+
+    int eval_expression(const shared_ptr<Ast> ast, shared_ptr<Environment> env) {
+        // expression <- sign term (term_op term)*
+        auto sign = ast->nodes[0]->token;
+        auto sign_val = (sign.empty() || sign == "+") ? 1 : -1;
+        auto val = eval(ast->nodes[1], env) * sign_val;
+        const auto& nodes = ast->nodes;
+        for (auto i = 2u; i < nodes.size(); i += 2) {
+            auto ope = nodes[i + 0]->token[0];
+            auto rval = eval(nodes[i + 1], env);
+            switch (ope) {
+                case '+':  val = val + rval; break;
+                case '-':  val = val - rval; break;
+            }
+        }
+        return val;
+    }
+
+    int eval_term(const shared_ptr<Ast> ast, shared_ptr<Environment> env) {
+        // term <- factor (factor_op factor)*
+        auto val = eval(ast->nodes[0], env);
+        const auto& nodes = ast->nodes;
+        for (auto i = 1u; i < nodes.size(); i += 2) {
+            auto ope = nodes[i + 0]->token[0];
+            auto rval = eval(nodes[i + 1], env);
+            switch (ope) {
+                case '*':
+                    val = val * rval;
+                    break;
+                case '/':
+                    if (rval == 0) {
+                        throw runtime_error("divide by 0 error");
+                    }
+                    val = val / rval;
+                    break;
+            }
+        }
+        return val;
+    }
+
+    int eval_ident(const shared_ptr<Ast> ast, shared_ptr<Environment> env) {
+        return env->get_value(ast->token);
+    }
+
+    int eval_number(const shared_ptr<Ast> ast, shared_ptr<Environment> env) {
+        return stol(ast->token);
+    }
+};
+
+bool read_file(const char* path, vector<char>& buff)
+{
+    ifstream ifs(path, ios::in | ios::binary);
+    if (ifs.fail()) {
+        return false;
+    }
+    buff.resize(static_cast<unsigned int>(ifs.seekg(0, ios::end).tellg()));
+    if (!buff.empty()) {
+        ifs.seekg(0, ios::beg).read(&buff[0], static_cast<streamsize>(buff.size()));
+    }
+    return true;
+}
+
+int main(int argc, const char** argv)
+{
+    if (argc < 2) {
+        cout << "usage: pl0 PATH [--ast]" << endl;
+        return 1;
+    }
+
+    auto path = argv[1];
+    vector<char> source;
+    if (!read_file(path, source)) {
+        cerr << "can't open the source file." << endl;
+        return -1;
+    }
+
+    peg parser(grammar);
+    parser.enable_ast(false, { "program", "statement", "statements", "term", "factor" });
+
+    shared_ptr<Ast> ast;
+    if (parser.parse_n(source.data(), source.size(), ast, path)) {
+        if (argc > 2 && string("--ast") == argv[2]) {
+            ast->print();
+        }
+        Interpreter interp;
+        auto env = make_shared<Environment>();
+        interp.exec(ast, env);
+        return 0;
+    }
+
+    cout << "syntax error..." << endl;
+    return -1;
+}
+
+// vim: et ts=4 sw=4 cin cino={1s ff=unix
diff --git a/example/pl0.vcxproj b/example/pl0.vcxproj
new file mode 100644
index 0000000..c746807
--- /dev/null
+++ b/example/pl0.vcxproj
@@ -0,0 +1,92 @@
+<?xml version="1.0" encoding="utf-8"?>
+<Project DefaultTargets="Build" ToolsVersion="14.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <ItemGroup Label="ProjectConfigurations">
+    <ProjectConfiguration Include="Debug|Win32">
+      <Configuration>Debug</Configuration>
+      <Platform>Win32</Platform>
+    </ProjectConfiguration>
+    <ProjectConfiguration Include="Release|Win32">
+      <Configuration>Release</Configuration>
+      <Platform>Win32</Platform>
+    </ProjectConfiguration>
+  </ItemGroup>
+  <ItemGroup>
+    <ClInclude Include="..\peglib.h" />
+  </ItemGroup>
+  <ItemGroup>
+    <ClCompile Include="pl0.cc" />
+  </ItemGroup>
+  <PropertyGroup Label="Globals">
+    <ProjectGuid>{6C5633BD-3CAE-498E-B0C6-ED90A1A99C47}</ProjectGuid>
+    <Keyword>Win32Proj</Keyword>
+    <RootNamespace>sample</RootNamespace>
+    <ProjectName>pl0</ProjectName>
+  </PropertyGroup>
+  <Import Project="$(VCTargetsPath)\Microsoft.Cpp.Default.props" />
+  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|Win32'" Label="Configuration">
+    <ConfigurationType>Application</ConfigurationType>
+    <UseDebugLibraries>true</UseDebugLibraries>
+    <CharacterSet>Unicode</CharacterSet>
+    <PlatformToolset>v140</PlatformToolset>
+  </PropertyGroup>
+  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Release|Win32'" Label="Configuration">
+    <ConfigurationType>Application</ConfigurationType>
+    <UseDebugLibraries>false</UseDebugLibraries>
+    <WholeProgramOptimization>true</WholeProgramOptimization>
+    <CharacterSet>Unicode</CharacterSet>
+    <PlatformToolset>v140</PlatformToolset>
+  </PropertyGroup>
+  <Import Project="$(VCTargetsPath)\Microsoft.Cpp.props" />
+  <ImportGroup Label="ExtensionSettings">
+  </ImportGroup>
+  <ImportGroup Label="PropertySheets" Condition="'$(Configuration)|$(Platform)'=='Debug|Win32'">
+    <Import Project="$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props" Condition="exists('$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props')" Label="LocalAppDataPlatform" />
+  </ImportGroup>
+  <ImportGroup Label="PropertySheets" Condition="'$(Configuration)|$(Platform)'=='Release|Win32'">
+    <Import Project="$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props" Condition="exists('$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props')" Label="LocalAppDataPlatform" />
+  </ImportGroup>
+  <PropertyGroup Label="UserMacros" />
+  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|Win32'">
+    <LinkIncremental>true</LinkIncremental>
+  </PropertyGroup>
+  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Release|Win32'">
+    <LinkIncremental>false</LinkIncremental>
+  </PropertyGroup>
+  <ItemDefinitionGroup Condition="'$(Configuration)|$(Platform)'=='Debug|Win32'">
+    <ClCompile>
+      <PrecompiledHeader>
+      </PrecompiledHeader>
+      <WarningLevel>Level3</WarningLevel>
+      <Optimization>Disabled</Optimization>
+      <PreprocessorDefinitions>WIN32;_DEBUG;_CONSOLE;%(PreprocessorDefinitions)</PreprocessorDefinitions>
+      <AdditionalIncludeDirectories>..</AdditionalIncludeDirectories>
+    </ClCompile>
+    <Link>
+      <SubSystem>Console</SubSystem>
+      <GenerateDebugInformation>true</GenerateDebugInformation>
+      <AdditionalDependencies>Ws2_32.lib;%(AdditionalDependencies)</AdditionalDependencies>
+    </Link>
+  </ItemDefinitionGroup>
+  <ItemDefinitionGroup Condition="'$(Configuration)|$(Platform)'=='Release|Win32'">
+    <ClCompile>
+      <WarningLevel>Level3</WarningLevel>
+      <PrecompiledHeader>
+      </PrecompiledHeader>
+      <Optimization>MaxSpeed</Optimization>
+      <FunctionLevelLinking>true</FunctionLevelLinking>
+      <IntrinsicFunctions>true</IntrinsicFunctions>
+      <PreprocessorDefinitions>WIN32;NDEBUG;_CONSOLE;%(PreprocessorDefinitions)</PreprocessorDefinitions>
+      <AdditionalIncludeDirectories>..</AdditionalIncludeDirectories>
+    </ClCompile>
+    <Link>
+      <SubSystem>Console</SubSystem>
+      <GenerateDebugInformation>true</GenerateDebugInformation>
+      <EnableCOMDATFolding>true</EnableCOMDATFolding>
+      <OptimizeReferences>true</OptimizeReferences>
+      <AdditionalDependencies>Ws2_32.lib;%(AdditionalDependencies)</AdditionalDependencies>
+    </Link>
+  </ItemDefinitionGroup>
+  <Import Project="$(VCTargetsPath)\Microsoft.Cpp.targets" />
+  <ImportGroup Label="ExtensionTargets">
+  </ImportGroup>
+</Project>
\ No newline at end of file