diff --git a/luprex/cpp/core/drivenengine.cpp b/luprex/cpp/core/drivenengine.cpp
index 6198d431..6148ab6f 100644
--- a/luprex/cpp/core/drivenengine.cpp
+++ b/luprex/cpp/core/drivenengine.cpp
@@ -221,8 +221,7 @@ enum DrvAction {
     PLAY_NOTIFY_CLOSE,
     PLAY_NOTIFY_ACCEPT,
     PLAY_CALL_EVENT_UPDATE,
-    PLAY_INVOKE_LUA_CALL,
-    PLAY_INVOKE_LUA_SOURCE,
+    PLAY_INVOKE,
     PLAY_RELEASE,
 };
 
@@ -235,8 +234,7 @@ inline static const char *action_string(DrvAction act) {
     case PLAY_NOTIFY_CLOSE: return "PLAY_NOTIFY_CLOSE";
     case PLAY_NOTIFY_ACCEPT: return "PLAY_NOTIFY_ACCEPT";
     case PLAY_CALL_EVENT_UPDATE: return "PLAY_CALL_EVENT_UPDATE";
-    case PLAY_INVOKE_LUA_CALL: return "PLAY_INVOKE_LUA_CALL";
-    case PLAY_INVOKE_LUA_SOURCE: return "PLAY_INVOKE_LUA_SOURCE";
+    case PLAY_INVOKE: return "PLAY_INVOKE_LUA_CALL";
     case PLAY_RELEASE: return "PLAY_RELEASE";
     default: return "unknown";
     }
@@ -483,15 +481,10 @@ void DrivenEngine::drv_call_event_update(double clock) {
     event_update();
 }
 
-void DrivenEngine::drv_invoke_lua_call(int64_t place, uint32_t datapklen, const char *datapk) {
-    Invocation *inv = new Invocation(Invocation::KIND_LUA_CALL, visible_actor_id_, place, std::string_view(datapk, datapklen));
+void DrivenEngine::drv_invoke(InvocationKind kind, int64_t place, uint32_t datapklen, const char *datapk) {
+    Invocation *inv = new Invocation(kind, visible_actor_id_, place, std::string_view(datapk, datapklen));
     queued_invocations_.emplace_back(inv);
-}
-
-void DrivenEngine::drv_invoke_lua_source(uint32_t srcpklen, const char *srcpk) {
-    Invocation *inv = new Invocation(Invocation::KIND_LUA_SOURCE, visible_actor_id_, visible_actor_id_, std::string_view(srcpk, srcpklen));
-    queued_invocations_.emplace_back(inv);
-    rescan_lua_source_ = false;
+    if (kind == InvocationKind::LUA_SOURCE) rescan_lua_source_ = false;
 }
 
 //////////////////////////////////////////////////////////////////////////////
@@ -818,49 +811,28 @@ static void replay_invoke_event_update(EngineWrapper *w) {
 ////////////////////////
 
 
-void play_invoke_lua_call(EngineWrapper *w, int64_t place, uint32_t datapklen, const char *datapk) {
+void play_invoke(EngineWrapper *w, InvocationKind kind, int64_t place, uint32_t datapklen, const char *datapk) {
     assert(w->rlog == nullptr);
     if (w->wlog != nullptr) {
-        w->wlog->write_cmd_hash(PLAY_INVOKE_LUA_CALL, eng::memhash());
+        w->wlog->write_cmd_hash(PLAY_INVOKE, eng::memhash());
+        w->wlog->write_uint8(int64_t(kind));
         w->wlog->write_int64(place);
         w->wlog->write_string(std::string_view(datapk, datapklen));
         w->wlog->flush();
     }
 
-    w->engine->drv_invoke_lua_call(place, datapklen, datapk);
+    w->engine->drv_invoke(kind, place, datapklen, datapk);
 }
 
-void replay_invoke_lua_call(EngineWrapper *w) {
+void replay_invoke(EngineWrapper *w) {
+    InvocationKind kind = InvocationKind(w->rlog->read_uint8());
     int64_t place = w->rlog->read_int64();
     std::string srcpack = w->rlog->read_string();
     if (!w->rlog->good()) {
         return reset_wrapper(w, "replay log corrupt in replay_invoke_lua_call");
     }
 
-    w->engine->drv_invoke_lua_call(place, srcpack.size(), srcpack.c_str());
-}
-
-////////////////////////
-
-
-void play_invoke_lua_source(EngineWrapper *w, uint32_t srcpklen, const char *srcpk) {
-    assert(w->rlog == nullptr);
-    if (w->wlog != nullptr) {
-        w->wlog->write_cmd_hash(PLAY_INVOKE_LUA_SOURCE, eng::memhash());
-        w->wlog->write_string(std::string_view(srcpk, srcpklen));
-        w->wlog->flush();
-    }
-
-    w->engine->drv_invoke_lua_source(srcpklen, srcpk);
-}
-
-void replay_invoke_lua_source(EngineWrapper *w) {
-    std::string srcpack = w->rlog->read_string();
-    if (!w->rlog->good()) {
-        return reset_wrapper(w, "replay log corrupt in replay_invoke_lua_source");
-    }
-
-    w->engine->drv_invoke_lua_source(srcpack.size(), srcpack.c_str());
+    w->engine->drv_invoke(kind, place, srcpack.size(), srcpack.c_str());
 }
 
 
@@ -931,8 +903,7 @@ static void replaycore_step(EngineWrapper *w) {
     case PLAY_NOTIFY_CLOSE: replay_notify_close(w); return;
     case PLAY_NOTIFY_ACCEPT: replay_notify_accept(w); return;
     case PLAY_CALL_EVENT_UPDATE: replay_invoke_event_update(w); return;
-    case PLAY_INVOKE_LUA_CALL: replay_invoke_lua_call(w); return;
-    case PLAY_INVOKE_LUA_SOURCE: replay_invoke_lua_source(w); return;
+    case PLAY_INVOKE: replay_invoke(w); return;
     case PLAY_RELEASE: release(w); return;
     default: return reset_wrapper(w, "Replay log corrupt in command dispatcher");
     }
@@ -993,8 +964,7 @@ static void init_engine_wrapper_helper(EngineWrapper *w) {
     w->play_notify_close = play_notify_close;
     w->play_notify_accept = play_notify_accept;
     w->play_invoke_event_update = play_invoke_event_update;
-    w->play_invoke_lua_call = play_invoke_lua_call;
-    w->play_invoke_lua_source = play_invoke_lua_source;
+    w->play_invoke = play_invoke;
 
     w->replay_initialize = replaycore_initialize;
     w->replay_step = replaycore_step;
diff --git a/luprex/cpp/core/drivenengine.hpp b/luprex/cpp/core/drivenengine.hpp
index 4521153f..880ea53c 100644
--- a/luprex/cpp/core/drivenengine.hpp
+++ b/luprex/cpp/core/drivenengine.hpp
@@ -152,6 +152,11 @@ public:
     //
     virtual void event_update() = 0;
 
+    // // The probe callback.  You may override this in a subclass.
+    // // This will be called whenever the driver wants to probe the state of the world.
+    // //
+    // virtual void event_probe(InvocationKind kind, int64_t place, std::string_view datapack, StreamBuffer *retvals) = 0;
+
     // Specify the set of listening ports.
     // This can only be used during the init routine.
     //
@@ -293,8 +298,7 @@ public:
     void drv_notify_close(uint32_t chid, uint32_t len, const char *data);
     uint32_t drv_notify_accept(uint32_t port);
     void drv_call_event_update(double clock);
-    void drv_invoke_lua_call(int64_t place, uint32_t datapklen, const char *datapk);
-    void drv_invoke_lua_source(uint32_t srcpklen, const char *srcpk);
+    void drv_invoke(InvocationKind kind, int64_t place, uint32_t datapklen, const char *datapk);
 
 private:
     // Find a currently-unused channel ID.  Channel IDs
diff --git a/luprex/cpp/core/enginewrapper.hpp b/luprex/cpp/core/enginewrapper.hpp
index e461d27c..900030e4 100644
--- a/luprex/cpp/core/enginewrapper.hpp
+++ b/luprex/cpp/core/enginewrapper.hpp
@@ -20,6 +20,15 @@
 #define DRV_ERRMSG_SIZE 8192
 #define DRV_SHORTSTRING_SIZE 65536
 
+enum class InvocationKind {
+    INVALID,
+    LUA_CALL,
+    LUA_EXPR,
+    LUA_SOURCE,
+    FLUSH_PRINTS,
+    TICK,
+};
+
 class DrivenEngine;
 class PlayLogfile;
 class ReplayLogfile;
@@ -197,21 +206,12 @@ struct EngineWrapper {
     //
     void (*play_invoke_event_update)(EngineWrapper *w, double clock);
 
-    // Send an invoke/KIND_LUA_CALL
+    // Send an invoke.
     //
     // This is the main pathway for blueprints, or the unreal C++ code,
-    // to reach into lua.  The datapack mus contain a classname, a function
-    // name, and then arguments for the function.
+    // to change the state of the world.
     //
-    void (*play_invoke_lua_call)(EngineWrapper *w, int64_t place, uint32_t datapklen, const char *datapk);
-
-    // Send an invoke/KIND_LUA_SOURCE
-    //
-    // This is used to update the lua source at runtime, without stopping
-    // the game.  In addition to sending the invoke/KIND_LUA_SOURCE, it also
-    // clears the rescan_lua_source flag.
-    //
-    void (*play_invoke_lua_source)(EngineWrapper *w, uint32_t srcpklen, const char *srcpk); 
+    void (*play_invoke)(EngineWrapper *w, InvocationKind kind, int64_t place, uint32_t datapklen, const char *datapk);
 
     //////////////////////////////////////////////////////////////////////////////
     //////////////////////////////////////////////////////////////////////////////
diff --git a/luprex/cpp/core/invocation.cpp b/luprex/cpp/core/invocation.cpp
index 4e2d1839..155f6f91 100644
--- a/luprex/cpp/core/invocation.cpp
+++ b/luprex/cpp/core/invocation.cpp
@@ -6,21 +6,21 @@
 
 #include "invocation.hpp"
 
-Invocation::Invocation() : kind_(KIND_INVALID), actor_(0), place_(0) {}
+Invocation::Invocation() : kind_(InvocationKind::INVALID), actor_(0), place_(0) {}
 
-Invocation::Invocation(Kind kind, int64_t actor, int64_t place, std::string_view datapack) 
+Invocation::Invocation(InvocationKind kind, int64_t actor, int64_t place, std::string_view datapack) 
     : kind_(kind), actor_(actor), place_(place), datapack_(datapack) {}
     
 
 void Invocation::serialize(StreamBuffer *sb) const {
-    sb->write_uint8(kind_);
+    sb->write_uint8(int64_t(kind_));
     sb->write_int64(actor_);
     sb->write_int64(place_);
     sb->write_string(datapack_);
 }
 
 void Invocation::deserialize(StreamBuffer *sb) {
-    kind_ = Kind(sb->read_uint8());
+    kind_ = InvocationKind(sb->read_uint8());
     actor_ = sb->read_int64();
     place_ = sb->read_int64();
     datapack_ = sb->read_string();
@@ -30,12 +30,12 @@ eng::string Invocation::debug_string() const {
     eng::ostringstream oss;
     oss << "inv[";
     switch (kind_) {
-    case KIND_INVALID: oss << "invalid"; break;
-    case KIND_LUA_CALL: oss << "lua_call"; break;
-    case KIND_LUA: oss << "lua"; break;
-    case KIND_FLUSH_PRINTS: oss << "flush_prints"; break;
-    case KIND_TICK: oss << "tick"; break;
-    case KIND_LUA_SOURCE: oss << "lua_source"; break;
+    case InvocationKind::INVALID: oss << "invalid"; break;
+    case InvocationKind::LUA_CALL: oss << "lua_call"; break;
+    case InvocationKind::LUA_EXPR: oss << "lua"; break;
+    case InvocationKind::FLUSH_PRINTS: oss << "flush_prints"; break;
+    case InvocationKind::TICK: oss << "tick"; break;
+    case InvocationKind::LUA_SOURCE: oss << "lua_source"; break;
     default: oss << "UNKNOWN"; break;
     }
     oss << " a=" << actor_;
diff --git a/luprex/cpp/core/invocation.hpp b/luprex/cpp/core/invocation.hpp
index 5af94798..3ade9986 100644
--- a/luprex/cpp/core/invocation.hpp
+++ b/luprex/cpp/core/invocation.hpp
@@ -8,28 +8,28 @@
 //
 // The actual contents of the datapack depends on the type of invocation:
 //
-// KIND_INVALID:
+// InvocationKind::INVALID:
 //
 //   Nothing.
 //
-// KIND_LUA_CALL:
+// InvocationKind::LUA_CALL:
 //
 //   A lua function call.  The class name, the function name, and then
 //   the function arguments.
 //
-// KIND_LUA:
+// InvocationKind::LUA_EXPR:
 //
 //   A block of lua source code, in plaintext.
 //
-// KIND_FLUSH_PRINTS:
+// InvocationKind::FLUSH_PRINTS:
 //
 //   Line number in ascii.
 //
-// KIND_TICK:
+// InvocationKind::TICK:
 //
 //   Nothing.
 //
-// KIND_LUA_SOURCE:
+// InvocationKind::LUA_SOURCE:
 //
 //   Packaged lua sourcecode.  See drvutil::package_lua_source.
 //
@@ -42,30 +42,22 @@
 #include "wrap-map.hpp"
 #include "wrap-deque.hpp"
 #include "streambuffer.hpp"
+#include "enginewrapper.hpp"
 
 
 class Invocation : public eng::opnew {
 public:
-    enum Kind {
-        KIND_INVALID,
-        KIND_LUA_CALL,
-        KIND_LUA,
-        KIND_FLUSH_PRINTS,
-        KIND_TICK,
-        KIND_LUA_SOURCE,
-    };
-
 private:
-    Kind kind_;
+    InvocationKind kind_;
     int64_t actor_;
     int64_t place_;
     eng::string datapack_;
 public:
     Invocation();
-    Invocation(Kind kind, int64_t actor, int64_t place, std::string_view datapack);
+    Invocation(InvocationKind kind, int64_t actor, int64_t place, std::string_view datapack);
     
-    bool valid() const { return kind_ != KIND_INVALID; }
-    Kind kind() const { return kind_; }
+    bool valid() const { return kind_ != InvocationKind::INVALID; }
+    InvocationKind kind() const { return kind_; }
     int64_t actor() const { return actor_; }
     int64_t place() const { return place_; }
     const eng::string &datapack() const { return datapack_; }
diff --git a/luprex/cpp/core/lpxclient.cpp b/luprex/cpp/core/lpxclient.cpp
index 68b451a7..6b9dcf0d 100644
--- a/luprex/cpp/core/lpxclient.cpp
+++ b/luprex/cpp/core/lpxclient.cpp
@@ -116,7 +116,7 @@ public:
 
     void send_invocation(const Invocation &inv) {
         if (channel_ == nullptr) {
-            if ((!world_->is_authoritative()) && (inv.kind() == Invocation::KIND_LUA_SOURCE)) {
+            if ((!world_->is_authoritative()) && (inv.kind() == InvocationKind::LUA_SOURCE)) {
                 // We have a client model, but no client connection.  That means we're
                 // in the process of shutting down a client model.  The client model
                 // is supposed to linger until the lua source is reread.  Once we have
@@ -178,7 +178,7 @@ public:
     }
 
     virtual void do_luainvoke_command(std::string_view cmd) override {
-        send_invocation(Invocation(Invocation::KIND_LUA, actor_id_, actor_id_, cmd));
+        send_invocation(Invocation(InvocationKind::LUA_EXPR, actor_id_, actor_id_, cmd));
     }
 
     virtual void do_luaprobe_command(std::string_view cmd) override {
diff --git a/luprex/cpp/core/lpxserver.cpp b/luprex/cpp/core/lpxserver.cpp
index 47267c21..26ee9b98 100644
--- a/luprex/cpp/core/lpxserver.cpp
+++ b/luprex/cpp/core/lpxserver.cpp
@@ -134,7 +134,7 @@ public:
     }
 
     virtual void do_luainvoke_command(std::string_view cmd) override {
-        master_->invoke(Invocation(Invocation::KIND_LUA, admin_id_, admin_id_, cmd));
+        master_->invoke(Invocation(InvocationKind::LUA_EXPR, admin_id_, admin_id_, cmd));
     }
 
     virtual void do_luaprobe_command(std::string_view cmd) override {
@@ -243,7 +243,7 @@ public:
 
         // If the clock has advanced far enough, tick the master model.
         if (clock >= next_tick_) {
-            master_->invoke(Invocation(Invocation::KIND_TICK, 0, 0, ""));
+            master_->invoke(Invocation(InvocationKind::TICK, 0, 0, ""));
             for (UniqueClient &client : clients_) {
                 client->async_diff_ = true;
             }
diff --git a/luprex/cpp/core/printbuffer.cpp b/luprex/cpp/core/printbuffer.cpp
index c1daff88..90f05b25 100644
--- a/luprex/cpp/core/printbuffer.cpp
+++ b/luprex/cpp/core/printbuffer.cpp
@@ -195,7 +195,7 @@ bool PrintChanneler::channel(const PrintBuffer *printbuffer, std::ostream &ostre
 Invocation PrintChanneler::invocation(int64_t actor_id) {
     char buf[80];
     sprintf(buf, "%" PRId64, line_);
-    return Invocation(Invocation::KIND_FLUSH_PRINTS, actor_id, actor_id, buf);
+    return Invocation(InvocationKind::FLUSH_PRINTS, actor_id, actor_id, buf);
 }
 
 LuaDefine(unittests_printbuffer, "", "some unit tests") {
diff --git a/luprex/cpp/core/world-core.cpp b/luprex/cpp/core/world-core.cpp
index e4c5ed0b..cd771eb5 100644
--- a/luprex/cpp/core/world-core.cpp
+++ b/luprex/cpp/core/world-core.cpp
@@ -700,19 +700,19 @@ void World::run_unittests() {
 
 void World::invoke(const Invocation &inv) {
     switch (inv.kind()) {
-    case Invocation::KIND_LUA_CALL:
+    case InvocationKind::LUA_CALL:
         invoke_lua_call(inv.actor(), inv.place(), inv.datapack());
         break;
-    case Invocation::KIND_LUA:
-        invoke_lua(inv.actor(), inv.place(), inv.datapack());
+    case InvocationKind::LUA_EXPR:
+        invoke_lua_expr(inv.actor(), inv.place(), inv.datapack());
         break;
-    case Invocation::KIND_FLUSH_PRINTS:
+    case InvocationKind::FLUSH_PRINTS:
         invoke_flush_prints(inv.actor(), inv.place(), inv.datapack());
         break;
-    case Invocation::KIND_TICK:
+    case InvocationKind::TICK:
         invoke_tick(inv.actor(), inv.place(), inv.datapack());
         break;
-    case Invocation::KIND_LUA_SOURCE:
+    case InvocationKind::LUA_SOURCE:
         invoke_lua_source(inv.actor(), inv.place(), inv.datapack());
         break;
     default:
@@ -827,7 +827,7 @@ void World::invoke_flush_prints(int64_t actor_id, int64_t place_id, std::string_
     assert(stack_is_clear());
 }
 
-void World::invoke_lua(int64_t actor_id, int64_t place_id, std::string_view datapack) {
+void World::invoke_lua_expr(int64_t actor_id, int64_t place_id, std::string_view datapack) {
     assert(stack_is_clear());
     {
         lua_State *L = state();
diff --git a/luprex/cpp/core/world.hpp b/luprex/cpp/core/world.hpp
index 731e92f3..fe6e8a0a 100644
--- a/luprex/cpp/core/world.hpp
+++ b/luprex/cpp/core/world.hpp
@@ -403,7 +403,7 @@ private:
     
     // Invoke a lua string.
     //
-    void invoke_lua(int64_t actor_id, int64_t place_id, std::string_view datapack);
+    void invoke_lua_expr(int64_t actor_id, int64_t place_id, std::string_view datapack);
     
     // Invoke the flush-prints operation.
     //
diff --git a/luprex/cpp/drv/driver.cpp b/luprex/cpp/drv/driver.cpp
index 16778a2c..95eed416 100644
--- a/luprex/cpp/drv/driver.cpp
+++ b/luprex/cpp/drv/driver.cpp
@@ -193,7 +193,7 @@ class Driver {
             std::string err = drvutil::package_lua_source(".", &oss);
             if_error_print_and_exit(err);
             std::string_view ossv = oss.view();
-            engw.play_invoke_lua_source(&engw, ossv.size(), ossv.data());
+            engw.play_invoke(&engw, InvocationKind::LUA_SOURCE, 0, ossv.size(), ossv.data());
         }
     }