diff --git a/example/CMakeLists.txt b/example/CMakeLists.txt
index 74704bb..0443aba 100644
--- a/example/CMakeLists.txt
+++ b/example/CMakeLists.txt
@@ -17,3 +17,6 @@ target_link_libraries(calc4 ${add_link_deps})
 
 add_executable(calc5 calc5.cc)
 target_link_libraries(calc5 ${add_link_deps})
+
+add_executable(indent indent.cc)
+target_link_libraries(indent ${add_link_deps})
diff --git a/example/indent.cc b/example/indent.cc
new file mode 100644
index 0000000..aec502f
--- /dev/null
+++ b/example/indent.cc
@@ -0,0 +1,67 @@
+//
+//  indent.cc
+//
+//  Copyright (c) 2022 Yuji Hirose. All rights reserved.
+//  MIT License
+//
+
+// Based on https://gist.github.com/dmajda/04002578dd41ae8190fc
+
+#include <cstdlib>
+#include <iostream>
+#include <peglib.h>
+
+using namespace peg;
+
+int main(void) {
+  parser parser(R"(Start <- Statements {}
+Statements <- Statement*
+Statement <- Samedent (S / I)
+
+S <- 'S' EOS { no_ast_opt }
+I <- 'I' EOL Block / 'I' EOS { no_ast_opt }
+
+Block <- Statements {}
+
+~Samedent <- ' '* {}
+
+~EOS <- EOL / EOF
+~EOL <- '\n'
+~EOF <- !.
+  )");
+
+  size_t indent = 0;
+
+  parser["Block"].enter = [&](const char * /*s*/, size_t /*n*/,
+                              std::any & /*dt*/) { indent += 2; };
+
+  parser["Block"].leave = [&](const char * /*s*/, size_t /*n*/,
+                              size_t /*matchlen*/, std::any & /*value*/,
+                              std::any & /*dt*/) { indent -= 2; };
+
+  parser["Samedent"] = [&](const SemanticValues &vs, std::any & /*dt*/) {
+    if (indent != vs.sv().size()) { throw parse_error("different indent..."); }
+  };
+
+  parser.enable_ast();
+
+  const auto source = R"(I
+  S
+  I
+    I
+      S
+      S
+    S
+  S
+)";
+
+  std::shared_ptr<Ast> ast;
+  if (parser.parse(source, ast)) {
+    ast = parser.optimize_ast(ast);
+    std::cout << ast_to_s(ast);
+    return 0;
+  }
+
+  std::cout << "syntax error..." << std::endl;
+  return -1;
+}