diff --git a/luprex/cpp/core/animqueue.cpp b/luprex/cpp/core/animqueue.cpp
index 76c773b0..4a19fe5a 100644
--- a/luprex/cpp/core/animqueue.cpp
+++ b/luprex/cpp/core/animqueue.cpp
@@ -323,8 +323,8 @@ bool AnimStep::from_string(const eng::string &config) {
     return true;
 }
 
-AnimQueue::AnimQueue(util::WorldType wt) {
-    version_autoinc_ = (wt == util::WORLD_TYPE_MASTER);
+AnimQueue::AnimQueue(WorldType wt) {
+    version_autoinc_ = (wt == WORLD_TYPE_MASTER);
     size_limit_ = 10; // Default size limit.
     steps_.emplace_back();
     steps_.front().keep_state_only();
@@ -576,8 +576,8 @@ static bool diff_works(const AnimQueue &master, AnimQueue &sync) {
 LuaDefine(unittests_animqueue, "", "some unit tests") {
     // Useful objects.
     AnimStep stp;
-    AnimQueue aq(util::WORLD_TYPE_MASTER);
-    AnimQueue aqds(util::WORLD_TYPE_S_SYNC);
+    AnimQueue aq(WORLD_TYPE_MASTER);
+    AnimQueue aqds(WORLD_TYPE_PREDICTIVE);
     StreamBuffer sb;
     
     // Debug string of a newly initialized queue
diff --git a/luprex/cpp/core/animqueue.hpp b/luprex/cpp/core/animqueue.hpp
index f3aaf94f..e9d16433 100644
--- a/luprex/cpp/core/animqueue.hpp
+++ b/luprex/cpp/core/animqueue.hpp
@@ -169,7 +169,7 @@ private:
 class AnimQueue : public eng::nevernew {
 public:
     // World type determines whether versions increment or autozero
-    AnimQueue(util::WorldType wt);
+    AnimQueue(WorldType wt);
         
     // Simple getters.
     const AnimStep &nth(int n) const { return steps_[n]; }
diff --git a/luprex/cpp/core/eng-tests.cpp b/luprex/cpp/core/eng-tests.cpp
index f21186a7..e4f17ba5 100644
--- a/luprex/cpp/core/eng-tests.cpp
+++ b/luprex/cpp/core/eng-tests.cpp
@@ -114,7 +114,7 @@ private:
 
     void event_init(int argc, char *argv[])
     {
-        world_.reset(new World(util::WORLD_TYPE_STANDALONE));
+        world_.reset(new World(WORLD_TYPE_MASTER));
         world_->update_source(get_lua_source());
         world_->run_unittests();
         stop_driver();
diff --git a/luprex/cpp/core/globaldb.cpp b/luprex/cpp/core/globaldb.cpp
index d90367f1..e2de6a1a 100644
--- a/luprex/cpp/core/globaldb.cpp
+++ b/luprex/cpp/core/globaldb.cpp
@@ -7,6 +7,8 @@ LuaDefine(global_once, "name", "for a given string, returns true exactly once")
     LuaVar oncedb, val;
     LuaStack LS(L, name, flag, oncedb, val);
 
+    LS.guard_nopredict("global.once");
+
     // Get a pointer to the oncedb.
     LS.rawget(oncedb, LuaRegistry, "oncedb");
     if (!LS.istable(oncedb)) {
@@ -25,11 +27,14 @@ LuaDefine(global_once, "name", "for a given string, returns true exactly once")
     return LS.result();
 }
 
+
 LuaDefine(global_clearonce, "name", "reset the specified once-flag") {
     LuaArg name;
     LuaVar oncedb;
     LuaStack LS(L, name, oncedb);
 
+    LS.guard_nopredict("global.clearonce");    
+
     // Get a pointer to the oncedb.
     LS.rawget(oncedb, LuaRegistry, "oncedb");
     if (!LS.istable(oncedb)) {
@@ -50,11 +55,6 @@ LuaDefine(global_table, "globalname", "get a table where global data can be stor
     // Get a pointer to the globaldb.
     LS.rawget(globaldb, LuaRegistry, "globaldb");
 
-    // nopredict
-    if (lua_isyieldable(L) && (!LS.istable(globaldb))) {
-        return lua_yield(L, 0);
-    }
-
     // Get the globaltab from the globaldb, sanity check it.
     LS.rawget(globaltab, globaldb, globalname);
     if (LS.istable(globaltab)) {
diff --git a/luprex/cpp/core/lpxclient.cpp b/luprex/cpp/core/lpxclient.cpp
index 811d97eb..02d1d345 100644
--- a/luprex/cpp/core/lpxclient.cpp
+++ b/luprex/cpp/core/lpxclient.cpp
@@ -24,7 +24,7 @@ public:
 public:
     void set_initial_state() {
         // Create the world model.
-        world_.reset(new World(util::WORLD_TYPE_C_SYNC));
+        world_.reset(new World(WORLD_TYPE_PREDICTIVE));
 
         // This is a temporary actor that will be used only until the server sends
         // us the first difference transmission.  We do this only to establish
diff --git a/luprex/cpp/core/lpxserver.cpp b/luprex/cpp/core/lpxserver.cpp
index 27dffcc4..9bdec15b 100644
--- a/luprex/cpp/core/lpxserver.cpp
+++ b/luprex/cpp/core/lpxserver.cpp
@@ -33,7 +33,7 @@ public:
 public:
     virtual void event_init(int argc, char *argv[]) {
         // Create the master world model.
-        master_.reset(new World(util::WORLD_TYPE_MASTER));
+        master_.reset(new World(WORLD_TYPE_MASTER));
 
         // Update the source code of the master model.
         master_->update_source(get_lua_source());
@@ -189,7 +189,7 @@ public:
                 Client *client = new Client;
                 client->actor_id_ = master_->create_login_actor();
                 client->channel_ = std::move(chan);
-                client->sync_.reset(new World(util::WORLD_TYPE_S_SYNC));
+                client->sync_.reset(new World(WORLD_TYPE_PREDICTIVE));
                 client->sync_->create_login_actor();
                 clients_.emplace_back(client);
                 stdostream() << "New client: actor id=" << client->actor_id_ << std::endl;
diff --git a/luprex/cpp/core/luasnap.cpp b/luprex/cpp/core/luasnap.cpp
index beb39d59..ae307e71 100644
--- a/luprex/cpp/core/luasnap.cpp
+++ b/luprex/cpp/core/luasnap.cpp
@@ -85,7 +85,8 @@ void LuaSnap::deserialize(StreamBuffer *sb) {
     void *ud = sb->lua_reader_ud(len);
 
     // Call eris with the permanents table and passing the snapshot as a lua_Reader.
-    lua_getfield(state_, LUA_REGISTRYINDEX, "unpersist");
+    lua_pushstring(state_, "unpersist");
+    lua_rawget(state_, LUA_REGISTRYINDEX);
     eris_undump(state_, sb->lua_reader, ud);
     assert(lua_gettop(state_) == 2);
 
diff --git a/luprex/cpp/core/luastack.cpp b/luprex/cpp/core/luastack.cpp
index 919e9301..1f30cf7b 100644
--- a/luprex/cpp/core/luastack.cpp
+++ b/luprex/cpp/core/luastack.cpp
@@ -466,6 +466,24 @@ void LuaStack::setvisited(LuaSlot tab, bool visited) const {
     lua_modflagbits(L_, tab.index(), 0x0010, visited ? 0x0010 : 0);
 }
 
+WorldType LuaStack::world_type() const {
+    lua_pushstring(L_, "worldtype");
+    lua_rawget(L_, LUA_REGISTRYINDEX);
+    lua_Integer n = lua_tointeger(L_, -1);
+    lua_pop(L_, 1);
+    assert(n != 0);
+    return (WorldType)n;
+}
+
+void LuaStack::guard_nopredict(const char *fn) {
+    if (lua_isyieldable(L_)) {
+        if (!is_authoritative()) {
+            lua_yield(L_, 0);
+            luaL_error(L_, "unexplained nopredict failure in %s", fn);
+        }
+    }
+}
+
 LuaKeywordParser::LuaKeywordParser(lua_State *L, int slot) {
     L_ = L;
     slot_ = slot;
diff --git a/luprex/cpp/core/luastack.hpp b/luprex/cpp/core/luastack.hpp
index 6600e14e..2460514f 100644
--- a/luprex/cpp/core/luastack.hpp
+++ b/luprex/cpp/core/luastack.hpp
@@ -226,6 +226,13 @@ int LuaTypeTagValue(lua_State *L) { return 0; }
 #define LUA_TT_GLOBALDB      22
 #define LUA_TT_CLASS         23
 
+// World types enum.
+
+enum WorldType {
+    WORLD_TYPE_MASTER = 1,
+    WORLD_TYPE_PREDICTIVE = 2,
+};
+
 // We use lightuserdata to store 'tokens': short
 // strings of 8 characters or less.  These tokens
 // are useful as unique markers.  The 8 characters
@@ -517,6 +524,17 @@ public:
     bool getvisited(LuaSlot tab) const;
     void setvisited(LuaSlot tab, bool visited) const;
 
+    // Return the world type (from the registry).
+    WorldType world_type() const;
+
+    // World types that are authoritative.
+    static bool is_authoritative(WorldType t) { return (t == WORLD_TYPE_MASTER); }
+    bool is_authoritative() { return is_authoritative(world_type()); }
+
+    // Stop execution of this thread if in a nonauth model,
+    // and if the thread is not a probe.
+    void guard_nopredict(const char *fn);
+
     // Return true if the int64 value can be stored as a lua number.
     static bool int64_storable(int64_t v) { return (v <= MAXINT) && (v >= -MAXINT); }
 };
diff --git a/luprex/cpp/core/textgame.cpp b/luprex/cpp/core/textgame.cpp
index cfac55c2..7fe96480 100644
--- a/luprex/cpp/core/textgame.cpp
+++ b/luprex/cpp/core/textgame.cpp
@@ -98,7 +98,7 @@ private:
 
     void event_init(int argc, char *argv[])
     {
-        world_.reset(new World(util::WORLD_TYPE_STANDALONE));
+        world_.reset(new World(WORLD_TYPE_MASTER));
         world_->update_source(get_lua_source());
         world_->run_unittests();
         actor_id_ = world_->create_login_actor();
diff --git a/luprex/cpp/core/util.hpp b/luprex/cpp/core/util.hpp
index a8fb5a7c..55b57f9f 100644
--- a/luprex/cpp/core/util.hpp
+++ b/luprex/cpp/core/util.hpp
@@ -198,13 +198,6 @@ bool valid_number(string_view v, bool plus, bool minus, bool dec, bool exp);
 
 namespace util {
 
-enum WorldType {
-    WORLD_TYPE_STANDALONE,
-    WORLD_TYPE_C_SYNC,
-    WORLD_TYPE_S_SYNC,
-    WORLD_TYPE_MASTER,
-};
-
 enum MessageType {
     MSG_NULL,
     MSG_DIFF,
diff --git a/luprex/cpp/core/world-core.cpp b/luprex/cpp/core/world-core.cpp
index 64122f91..87e5e676 100644
--- a/luprex/cpp/core/world-core.cpp
+++ b/luprex/cpp/core/world-core.cpp
@@ -29,12 +29,12 @@ World *World::fetch_global_pointer(lua_State *L) {
 World::~World() {
 }
 
-World::World(util::WorldType wt) {
+World::World(WorldType wt) {
     // Master world model by default.
     world_type_ = wt;
 
     // Initialize the ID allocator in master mode.
-    if (wt == util::WORLD_TYPE_MASTER || wt == util::WORLD_TYPE_STANDALONE) {
+    if (is_authoritative()) {
         id_global_pool_.init_master();
     } else {
         id_global_pool_.init_synch();
@@ -63,11 +63,12 @@ World::World(util::WorldType wt) {
     // Create the tangibles table in the registry.
     LS.rawset(LuaRegistry, "tangibles", LuaNewTable);
 
+    // Store the world type in the registry.
+    LS.rawset(LuaRegistry, "worldtype", wt);
+        
     // Create the globaldb and oncedb in the registry.
-    if ((wt == util::WORLD_TYPE_MASTER) || (wt == util::WORLD_TYPE_STANDALONE)) {
-        LS.rawset(LuaRegistry, "globaldb", LuaNewTable);
-        LS.rawset(LuaRegistry, "oncedb", LuaNewTable);
-    }
+    LS.rawset(LuaRegistry, "globaldb", LuaNewTable);
+    LS.rawset(LuaRegistry, "oncedb", LuaNewTable);
 
     // Initialize the SourceDB.  At this stage, the sourcedb is
     // empty, so it's just populating the lua builtins.
@@ -788,12 +789,12 @@ void World::invoke_lua_source(int64_t actor_id, int64_t place_id, const eng::str
 
 void World::guard_blockable(lua_State *L, const char *fn) {
     if (lthread_thread_id_ == 0) {
-        // in a probe, http.get throws an error.
+        // in a probe, blocking functions like http.get throw an error.
         luaL_error(L, "cannot %s in a probe", fn);
         assert(false);
     }
     if (!is_authoritative()) {
-        // in a nonauth model, http.get is converted to nopredict.
+        // in a nonauth model, blocking functions like http.get are converted to nopredict.
         lua_yield(L, 0);
         luaL_error(L, "unexplained nopredict failure in %s", fn);
         assert(false);
@@ -801,6 +802,8 @@ void World::guard_blockable(lua_State *L, const char *fn) {
 }
 
 void World::guard_nopredict(lua_State *L, const char *fn) {
+    // Caution: this code must be equivalent to the
+    // code in LuaStack::guard_nopredict.
     if (lthread_thread_id_ == 0) {
         return;
     }
@@ -883,7 +886,10 @@ void World::run_scheduled_threads() {
                 LS.rawset(thinfo, "isnew", false);
                 LS.rawset(thinfo, "useppool", false);
             } else {
-                // In a nonauth model, a yield is converted to a 'nopredict'.
+                // When a nonauthoritative model yields, for any reason,
+                // the thread is discarded.  This is also used as a way to implement
+                // nopredict: the thread that wants to 'nopredict' just yields,
+                // knowing that this will cause it to be killed.
                 LS.rawset(threads, sched.thread_id(), LuaNil);
             }
         } else {
diff --git a/luprex/cpp/core/world-pairtab.cpp b/luprex/cpp/core/world-pairtab.cpp
index f6aab82a..fc1b829b 100644
--- a/luprex/cpp/core/world-pairtab.cpp
+++ b/luprex/cpp/core/world-pairtab.cpp
@@ -136,8 +136,6 @@ void World::pair_lua_tables(const IdVector &basis, lua_State *master) {
         // If the master table is already paired, skip.
         MLS.rawget(midx, mtnmap, mtab);
         if (MLS.isnumber(midx)) continue;
-        // If the synch table is not a table, skip.
-        if (!SLS.istable(stab)) continue;
         // If the synch table doesn't have a number, skip.
         SLS.rawget(sidx, stnmap, stab);
         if (!SLS.isnumber(sidx)) continue;
diff --git a/luprex/cpp/core/world-testing.cpp b/luprex/cpp/core/world-testing.cpp
index 0d777d08..889fb748 100644
--- a/luprex/cpp/core/world-testing.cpp
+++ b/luprex/cpp/core/world-testing.cpp
@@ -241,9 +241,9 @@ static bool worlds_identical(const UniqueWorld &w1, const UniqueWorld &w2) {
 }
 
 LuaDefine(unittests_world1animdiff, "", "some unit tests") {
-    UniqueWorld m(new World(util::WORLD_TYPE_MASTER));
-    UniqueWorld ss(new World(util::WORLD_TYPE_S_SYNC));
-    UniqueWorld cs(new World(util::WORLD_TYPE_C_SYNC));
+    UniqueWorld m(new World(WORLD_TYPE_MASTER));
+    UniqueWorld ss(new World(WORLD_TYPE_PREDICTIVE));
+    UniqueWorld cs(new World(WORLD_TYPE_PREDICTIVE));
     StreamBuffer sb;
     util::IdVector ids = util::id_vector_create(123, 345);
 
@@ -311,8 +311,8 @@ LuaDefine(unittests_world1animdiff, "", "some unit tests") {
 }
 
 LuaDefine(unittests_world2pairtab, "", "some unit tests") {
-    UniqueWorld m(new World(util::WORLD_TYPE_MASTER));
-    UniqueWorld ss(new World(util::WORLD_TYPE_S_SYNC));
+    UniqueWorld m(new World(WORLD_TYPE_MASTER));
+    UniqueWorld ss(new World(WORLD_TYPE_PREDICTIVE));
     StreamBuffer sb;
     int ncreate;
 
@@ -359,9 +359,9 @@ LuaDefine(unittests_world2pairtab, "", "some unit tests") {
 }
 
 LuaDefine(unittests_world3diffluatab, "", "some unit tests") {
-    UniqueWorld m(new World(util::WORLD_TYPE_MASTER));
-    UniqueWorld ss(new World(util::WORLD_TYPE_S_SYNC));
-    UniqueWorld cs(new World(util::WORLD_TYPE_C_SYNC));
+    UniqueWorld m(new World(WORLD_TYPE_MASTER));
+    UniqueWorld ss(new World(WORLD_TYPE_PREDICTIVE));
+    UniqueWorld cs(new World(WORLD_TYPE_PREDICTIVE));
     StreamBuffer sb;
 
     // Initialize all three models so that a tangible exists.
@@ -413,9 +413,9 @@ LuaDefine(unittests_world3diffluatab, "", "some unit tests") {
 }
 
 LuaDefine(unittests_world4difftanclass, "", "some unit tests") {
-    UniqueWorld m(new World(util::WORLD_TYPE_MASTER));
-    UniqueWorld ss(new World(util::WORLD_TYPE_S_SYNC));
-    UniqueWorld cs(new World(util::WORLD_TYPE_C_SYNC));
+    UniqueWorld m(new World(WORLD_TYPE_MASTER));
+    UniqueWorld ss(new World(WORLD_TYPE_PREDICTIVE));
+    UniqueWorld cs(new World(WORLD_TYPE_PREDICTIVE));
     StreamBuffer sb;
 
     // Initialize all three models so that a tangible exists.
diff --git a/luprex/cpp/core/world.hpp b/luprex/cpp/core/world.hpp
index b506d91d..6c416105 100644
--- a/luprex/cpp/core/world.hpp
+++ b/luprex/cpp/core/world.hpp
@@ -104,7 +104,7 @@ public:
     // The constructor also calls 'lua_open' to create a new
     // lua interpreter for this world model.
     //
-    World(util::WorldType wt);
+    World(WorldType wt);
 
     // Destructor.
     //
@@ -236,8 +236,8 @@ public:
 
     // Check if the world is authoritative.
     //
-    bool is_authoritative() const { return (world_type_ == util::WORLD_TYPE_MASTER) || (world_type_ == util::WORLD_TYPE_STANDALONE); }
-
+    bool is_authoritative() const { return LuaStack::is_authoritative(world_type_); }
+    
     // Get a table showing all outstanding HTTP requests.
     //
     const HttpClientRequestMap &http_requests() const { return http_requests_; }
@@ -249,6 +249,9 @@ public:
 
     // Snapshot and rollback.
     //
+    // These are used by the client to convert the synchronous model
+    // to an asynchronous model and back.
+    //
     void snapshot();
     void rollback();
     bool snapshot_empty() { return snapshot_.empty(); }
@@ -494,7 +497,7 @@ public:
 
 private:
     // Type of model
-    util::WorldType world_type_;
+    WorldType world_type_;
 
     // A lua intepreter with snapshot function.
     //