diff --git a/luprex/core/cpp/luastack.hpp b/luprex/core/cpp/luastack.hpp
index 58da7010..e34e99eb 100644
--- a/luprex/core/cpp/luastack.hpp
+++ b/luprex/core/cpp/luastack.hpp
@@ -391,6 +391,8 @@ public:
     void makeclass(LuaSlot tab, const char *name) const;
     void makeclass(LuaSlot tab, const std::string &name) const;
     std::string classname(LuaSlot tab);
+    // There's no 'isclass' operator, but 'classname' will return empty
+    // string if tab is not a valid class.
 
     void movesortablekey(LuaSlot val, LuaStack &other, LuaSlot otherslot);
 
diff --git a/luprex/core/cpp/main.cpp b/luprex/core/cpp/main.cpp
index 9532a649..8b2df45d 100644
--- a/luprex/core/cpp/main.cpp
+++ b/luprex/core/cpp/main.cpp
@@ -126,7 +126,6 @@ public:
 
 int main(int argc, char **argv)
 {
-    //TextGame tg;
-    TNTest4 tg;
+    TextGame tg;
     driver_drive(&tg);
 }
diff --git a/luprex/core/cpp/textgame.cpp b/luprex/core/cpp/textgame.cpp
index d41c0899..6df69ad3 100644
--- a/luprex/core/cpp/textgame.cpp
+++ b/luprex/core/cpp/textgame.cpp
@@ -46,7 +46,9 @@ static void l_message(const char *msg)
 }
 
 void TextGame::do_lua(const std::string &exp) {
+    assert(world_->stack_is_clear());
     lua_State *L = world_->state();
+    // push the compiled function.
     int status = luaL_loadbuffer(L, exp.c_str(), exp.size(), "=stdin");
     assert(status == LUA_OK);
     globalL = L;
@@ -67,6 +69,7 @@ void TextGame::do_lua(const std::string &exp) {
         lua_pop(L, 1);
         lua_gc(L, LUA_GCCOLLECT, 0);
     }
+    assert(world_->stack_is_clear());
 }
 
 void TextGame::do_view_command(const StringVec &cmd) {
@@ -138,6 +141,17 @@ void TextGame::do_rollback_command(const StringVec &cmd) {
     world_->rollback();
 }
 
+void TextGame::do_tick_command(const StringVec &cmd) {
+    int64_t clock;
+    if (cmd.size() == 2) {
+        clock = util::strtoint(cmd[1], -1);
+    } else {
+        std::cerr << "t command (tick) expects a time value" << std::endl;
+        return;
+    }
+    world_->run_scheduled_threads(clock);
+}
+
 void TextGame::do_quit_command(const StringVec &cmd) {
     if (cmd.size() != 1) {
         std::cerr << "q command (quit) takes no arguments" << std::endl;
@@ -153,6 +167,7 @@ void TextGame::do_command(const StringVec &words) {
     else if (words[0] == "q") do_quit_command(words);
     else if (words[0] == "s") do_snapshot_command(words);
     else if (words[0] == "r") do_rollback_command(words);
+    else if (words[0] == "t") do_tick_command(words);
     else if (util::validinteger(words[0])) do_choose_command(words);
     else {
         std::cerr << "Unknown command: " << words[0] << std::endl;
diff --git a/luprex/core/cpp/textgame.hpp b/luprex/core/cpp/textgame.hpp
index 5e7b07da..bdc9ba93 100644
--- a/luprex/core/cpp/textgame.hpp
+++ b/luprex/core/cpp/textgame.hpp
@@ -22,6 +22,7 @@ private:
     void do_quit_command(const StringVec &cmd);
     void do_snapshot_command(const StringVec &cmd);
     void do_rollback_command(const StringVec &cmd);
+    void do_tick_command(const StringVec &cmd);
     
     void do_lua(const std::string &exp);
     void do_command(const StringVec &exp);
diff --git a/luprex/core/cpp/world-accessor.cpp b/luprex/core/cpp/world-accessor.cpp
index edd7e5ec..f7a31194 100644
--- a/luprex/core/cpp/world-accessor.cpp
+++ b/luprex/core/cpp/world-accessor.cpp
@@ -40,7 +40,11 @@ LuaDefine(tangible_setclass, "c") {
     LuaStack LS(L, tanobj, classname, classtab, mt);
     World *w = World::fetch_global_pointer(L);
     w->tangible_get(LS, tanobj);
-    LS.getclass(classtab, classname);
+    if (LS.classname(classname) != "") {
+        LS.set(classtab, classname);
+    } else {
+        LS.getclass(classtab, classname);
+    }
     LS.getmetatable(mt, tanobj);
     LS.rawset(mt, "__index", classtab);
     return LS.result();
diff --git a/luprex/core/cpp/world-core.cpp b/luprex/core/cpp/world-core.cpp
index 1cc30926..93fb7310 100644
--- a/luprex/core/cpp/world-core.cpp
+++ b/luprex/core/cpp/world-core.cpp
@@ -403,10 +403,14 @@ void World::invoke_plan(int64_t actor_id, int64_t place_id, const std::string &a
 
     // Create a new thread, set up function and parameters.
     lua_State *CO = LS.newthread(thread);
+    
+    // Push the function to be invoked and its arguments.
     lua_pushvalue(L, func.index());
     lua_pushvalue(L, actor.index());
     lua_pushvalue(L, place.index());
     lua_pushvalue(L, invdata.index());
+
+    // Move actor, place, function and args to new thread.
     lua_xmove(L, CO, 4);
 
     // Store the thread into place's thread table.
@@ -454,7 +458,7 @@ void World::run_scheduled_threads(int64_t clk) {
         // Resume the coroutine.
         lua_State *CO = LS.ckthread(thread);
         int top = lua_gettop(CO);
-        int status = lua_resume(CO, nullptr, (top > 0) ? (top - 1) : 0);
+        int status = lua_resume(CO, nullptr, (top >= 4) ? 3 : 0);
 
         // Three possible outcomes: finished, yielded, or errored.
         if (status == LUA_YIELD) {
diff --git a/luprex/core/cpp/world.hpp b/luprex/core/cpp/world.hpp
index b8812dd9..e180b969 100644
--- a/luprex/core/cpp/world.hpp
+++ b/luprex/core/cpp/world.hpp
@@ -199,19 +199,20 @@ public:
     void snapshot();
     void rollback();
 
-private:
     // Run any threads which according to the scheduler queue are ready.
     //
     void run_scheduled_threads(int64_t clk);
 
-    // Store a pointer to a world model into a lua registry.
-    //
-    static void store_global_pointer(lua_State *L, World *w);
-
     // Check that the main thread has nothing on the stack
     //
     bool stack_is_clear() const { return lua_gettop(state()) == 0; }
 
+private:
+
+    // Store a pointer to a world model into a lua registry.
+    //
+    static void store_global_pointer(lua_State *L, World *w);
+
     // Invoke a plan.
     //
     void invoke_plan(int64_t actor_id, int64_t place_id, const std::string &action, const InvocationData &idata);