diff --git a/CMakeLists.txt b/CMakeLists.txt
index 70df0a8..b2edcca 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -34,5 +34,30 @@ if(FE_ABSL)
     set(ABSL_ENABLE_INSTALL ON)
     find_package(abseil)
 endif()
+
 add_subdirectory(external/fe)
-add_subdirectory(src)
+
+add_executable(let)
+target_sources(let
+    PRIVATE
+        src/main.cpp
+        src/let/eval.cpp
+        src/let/lexer.cpp
+        src/let/parser.cpp
+        src/let/stream.cpp
+        src/let/tok.cpp
+        include/let/ast.h
+        include/let/driver.h
+        include/let/lexer.h
+        include/let/tok.h
+)
+target_link_libraries(let PRIVATE fe)
+target_include_directories(let
+    PUBLIC
+        $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/include>
+        $<INSTALL_INTERFACE:include>
+)
+target_compile_features(let PUBLIC cxx_std_${CMAKE_CXX_STANDARD})
+if (MSVC AND BUILD_SHARED_LIBS AND FE_ABSL)
+    target_compile_definitions(liblet PUBLIC ABSL_CONSUME_DLL)
+endif()
diff --git a/external/fe b/external/fe
index c26fe06..ad53460 160000
--- a/external/fe
+++ b/external/fe
@@ -1 +1 @@
-Subproject commit c26fe0620971c7f936e680258a7c40846be0ea5d
+Subproject commit ad53460a104cd0a8662d0481bac0a303542edbff
diff --git a/include/let/tok.h b/include/let/tok.h
index cfc2f07..add6dfb 100644
--- a/include/let/tok.h
+++ b/include/let/tok.h
@@ -44,6 +44,7 @@ class Tok {
 public:
     // clang-format off
     enum class Tag {
+        Nil,
 #define CODE(t, _) t,
         LET_KEY(CODE)
         LET_VAL(CODE)
@@ -80,6 +81,7 @@ class Tok {
     Tag tag() const { return tag_; }
     bool isa(Tag tag) const { return tag == tag_; }
     bool isa_key() const { return (int)tag() < Num_Keys; }
+    explicit operator bool() const { return tag_ != Tag::Nil; }
 
     Sym sym() const {
         assert(isa(Tag::V_sym));
@@ -96,7 +98,7 @@ class Tok {
 
 private:
     Loc loc_;
-    Tag tag_;
+    Tag tag_ = Tag::Nil;
     union {
         Sym sym_;
         uint64_t u64_;
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
deleted file mode 100644
index b572a89..0000000
--- a/src/CMakeLists.txt
+++ /dev/null
@@ -1,15 +0,0 @@
-add_executable(let main.cpp)
-
-target_sources(let PRIVATE main.cpp)
-target_link_libraries(let PRIVATE fe)
-target_include_directories(let
-    PUBLIC
-        $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/include>
-        $<INSTALL_INTERFACE:include>
-)
-target_compile_features(let PUBLIC cxx_std_${CMAKE_CXX_STANDARD})
-if (MSVC AND BUILD_SHARED_LIBS AND FE_ABSL)
-    target_compile_definitions(liblet PUBLIC ABSL_CONSUME_DLL)
-endif()
-
-add_subdirectory(let)
diff --git a/src/let/CMakeLists.txt b/src/let/CMakeLists.txt
deleted file mode 100644
index 8317735..0000000
--- a/src/let/CMakeLists.txt
+++ /dev/null
@@ -1,8 +0,0 @@
-target_sources(let
-    PRIVATE
-        eval.cpp
-        lexer.cpp
-        parser.cpp
-        stream.cpp
-        tok.cpp
-)