diff --git a/luprex/core/TODO b/luprex/core/TODO
index 111c2258..e93dc485 100644
--- a/luprex/core/TODO
+++ b/luprex/core/TODO
@@ -2,8 +2,6 @@ Calling out to external servers.
 
 Support ANSI escape sequences on output.
 
-Make math.random do something predictable.
-
 Do something about std::cerr && std::cout once and for all.
 
 Fix math.random
diff --git a/luprex/core/cpp/globaldb.cpp b/luprex/core/cpp/globaldb.cpp
index c85aa72f..d90367f1 100644
--- a/luprex/core/cpp/globaldb.cpp
+++ b/luprex/core/cpp/globaldb.cpp
@@ -1,7 +1,7 @@
 #include "luastack.hpp"
 #include "globaldb.hpp"
 
-LuaDefine(global_once, "string", "for a given string, returns true exactly once") {
+LuaDefine(global_once, "name", "for a given string, returns true exactly once") {
     LuaArg name;
     LuaRet flag;
     LuaVar oncedb, val;
@@ -14,7 +14,7 @@ LuaDefine(global_once, "string", "for a given string, returns true exactly once"
         return LS.result();
     }
 
-    LS.checkstring(name);
+    LS.checkstring(name, "name");
     LS.rawget(val, oncedb, name);
     if (!LS.isnil(val)) {
         LS.set(flag, false);
@@ -35,7 +35,7 @@ LuaDefine(global_clearonce, "name", "reset the specified once-flag") {
     if (!LS.istable(oncedb)) {
         return LS.result();
     }
-    LS.checkstring(name);
+    LS.checkstring(name, "name");
     LS.rawset(oncedb, name, LuaNil);
     return LS.result();
 }
@@ -45,7 +45,7 @@ LuaDefine(global_table, "globalname", "get a table where global data can be stor
     LuaRet globaltab;
     LuaVar globaldb;
     LuaStack LS(L, globalname, globaltab, globaldb);
-    LS.checkstring(globalname);
+    LS.checkstring(globalname, "globalname");
 
     // Get a pointer to the globaldb.
     LS.rawget(globaldb, LuaRegistry, "globaldb");
diff --git a/luprex/core/cpp/luastack.cpp b/luprex/core/cpp/luastack.cpp
index c938fa4b..7ba295e9 100644
--- a/luprex/core/cpp/luastack.cpp
+++ b/luprex/core/cpp/luastack.cpp
@@ -3,6 +3,8 @@
 #include <cassert>
 #include <cstdio>
 #include <climits>
+#include "wrap-string.hpp"
+#include "wrap-set.hpp"
 
 LuaSpecial LuaRegistry(LUA_REGISTRYINDEX);
 LuaNilMarker LuaNil;
@@ -76,6 +78,10 @@ lua_State *LuaStack::newstate (lua_Alloc allocf) {
     return L;
 }
 
+void LuaStack::argerr(const char *nm, const char *tp) const {
+    luaL_error(L_, "'%s' should be %s", nm, tp);
+}
+
 bool LuaStack::isinteger(LuaSlot s) const {
     if (lua_type(L_, s) == LUA_TNUMBER) {
         lua_Number result = lua_tonumber(L_, s);
@@ -93,11 +99,12 @@ bool LuaStack::isint(LuaSlot s) const {
 }
 
 bool LuaStack::ckboolean(LuaSlot s) const {
-    luaL_checktype(L_, s, LUA_TBOOLEAN);
+    checkboolean(s, "value");
     return lua_toboolean(L_, s) ? true:false;
 }
 
 lua_Integer LuaStack::ckinteger(LuaSlot s) const {
+    checknumber(s, "value");
     luaL_checktype(L_, s, LUA_TNUMBER);
     lua_Number result = lua_tonumber(L_, s);
     lua_Integer iresult(result);
@@ -109,7 +116,7 @@ lua_Integer LuaStack::ckinteger(LuaSlot s) const {
 }
 
 int LuaStack::ckint(LuaSlot s) const {
-    luaL_checktype(L_, s, LUA_TNUMBER);
+    checknumber(s, "value");
     lua_Number result = lua_tonumber(L_, s);
     int iresult(result);
     if (iresult != result) {
@@ -120,31 +127,31 @@ int LuaStack::ckint(LuaSlot s) const {
 }
 
 lua_Number LuaStack::cknumber(LuaSlot s) const {
-    luaL_checktype(L_, s, LUA_TNUMBER);
+    checknumber(s, "value");
     return lua_tonumber(L_, s);
 }
 
 eng::string LuaStack::ckstring(LuaSlot s) const {
-    luaL_checktype(L_, s, LUA_TSTRING);
+    checkstring(s, "value");
     size_t len;
     const char *str = lua_tolstring(L_, s, &len);
     return eng::string(str, len);
 }
 
 std::string_view LuaStack::ckstringview(LuaSlot s) const {
-    luaL_checktype(L_, s, LUA_TSTRING);
+    checkstring(s, "value");
     size_t len;
     const char *str = lua_tolstring(L_, s, &len);
     return std::string_view(str, len);
 }
 
 lua_State *LuaStack::ckthread(LuaSlot s) const {
-    luaL_checktype(L_, s, LUA_TTHREAD);
+    checkthread(s, "value");
     return lua_tothread(L_, s);
 }
 
 LuaToken LuaStack::cktoken(LuaSlot s) const {
-    luaL_checktype(L_, s, LUA_TLIGHTUSERDATA);
+    checktoken(s, "value");
     return LuaToken(lua_touserdata(L_, s));
 }
 
@@ -457,3 +464,41 @@ bool LuaStack::getvisited(LuaSlot tab) const {
 void LuaStack::setvisited(LuaSlot tab, bool visited) const {
     lua_modflagbits(L_, tab.index(), 0x0010, visited ? 0x0010 : 0);
 }
+
+LuaKeywordParser::LuaKeywordParser(lua_State *L, int slot) {
+    L_ = L;
+    slot_ = slot;
+    if (!lua_istable(L_, slot_)) {
+        luaL_error(L_, "expected an argument which is a table full of keywords");
+    }
+}
+
+bool LuaKeywordParser::parse(LuaSlot out, const char *kw) {
+    lua_pushstring(L_, kw);
+    lua_rawget(L_, slot_);
+    lua_replace(L_, out.index());
+    if (!lua_isnil(L_, out.index())) {
+        parsed_.insert(kw);
+        return true;
+    } else {
+        return false;
+    }
+};
+
+void LuaKeywordParser::check_unparsed_keywords() {
+    if (lua_nkeys(L_, slot_) != int(parsed_.size())) {
+        lua_pushnil(L_);
+        while (lua_next(L_, slot_) != 0) {
+            lua_pop(L_, 1); // Don't need the value.
+            if (!lua_isstring(L_, -1)) {
+                luaL_error(L_, "keyword table contains non-string key");
+            }
+            const char *kw = lua_tostring(L_, -1);
+            if (parsed_.find(kw) == parsed_.end()) {
+                luaL_error(L_, "keyword %s not known", kw);
+            }
+        }
+        assert(false && "should never get here in check_unparsed_keywords");
+    }
+}
+
diff --git a/luprex/core/cpp/planemap.cpp b/luprex/core/cpp/planemap.cpp
index cfefce3e..ae04f10c 100644
--- a/luprex/core/cpp/planemap.cpp
+++ b/luprex/core/cpp/planemap.cpp
@@ -827,90 +827,61 @@ void PlaneMap::untrack_all() {
     }
 }
 
-eng::string PlaneScan::configure(const LuaStack &LS0, LuaSlot config) {
+void PlaneScan::configure(LuaKeywordParser &kp) {
+    lua_State *L = kp.state();
     LuaVar val, vx, vy, vz;
-    LuaStack LS(LS0.state(), val, vx, vy, vz);
-    
-    if (!LS.istable(config)) {
-        return "scan configuration is not a table";
-    }
+    LuaStack LS(L, val, vx, vy, vz);
 
     bool have_plane = false;
     bool have_center = false;
     bool have_radius = false;
     bool have_shape = false;
     bool have_near = false;
-    int parameters = 0;
 
-    LS.rawget(val, config, "plane");
-    if (!LS.isnil(val)) {
-        if (!LS.isstring(val)) {
-            return "scan configuration: 'plane' must be a string";
-        }
+    if (kp.parse(val, "plane")) {
+        LS.checkstring(val, "plane");
         plane_ = LS.ckstring(val);
         have_plane = true;
-        parameters += 1;
     }
 
-    LS.rawget(vx, config, "centerx");
-    LS.rawget(vy, config, "centery");
-    LS.rawget(vz, config, "centerz");
+    kp.parse(vx, "centerx");
+    kp.parse(vy, "centery");
+    kp.parse(vz, "centerz");
     if ((!LS.isnil(vx)) || (!LS.isnil(vy)) || (!LS.isnil(vz))) {
-        if (!LS.isnumber(vx)) {
-            return "scan configuration: 'centerx' must be a number";
-        }
-        if (!LS.isnumber(vy)) {
-            return "scan configuration: 'centery' must be a number";
-        }
-        if (!LS.isnumber(vz)) {
-            return "scan configuration: 'centerz' must be a number";
-        }
+        LS.checknumber(vx, "centerx");
+        LS.checknumber(vy, "centery");
+        LS.checknumber(vz, "centerz");
         center_.x = LS.cknumber(vx);
         center_.y = LS.cknumber(vy);
         center_.z = LS.cknumber(vz);
         have_center = true;
-        parameters += 3;
     }
 
-    LS.rawget(val, config, "radius");
-    if (!LS.isnil(val)) {
-        if (!LS.isnumber(val)) {
-            return "scan configuration: 'radius' must be a number";
-        }
+    if (kp.parse(val, "radius")) {
+        LS.checknumber(val, "radius");
         radius_.x = LS.cknumber(val);
         radius_.y = radius_.z = radius_.x;
         have_radius = true;
-        parameters += 1;
     }
 
-    LS.rawget(vx, config, "radiusx");
-    LS.rawget(vy, config, "radiusy");
-    LS.rawget(vz, config, "radiusz");
+    kp.parse(vx, "radiusx");
+    kp.parse(vy, "radiusy");
+    kp.parse(vz, "radiusz");
     if ((!LS.isnil(vx)) || (!LS.isnil(vy)) || (!LS.isnil(vz))) {
-        if (!LS.isnumber(vx)) {
-            return "scan configuration: 'radiusx' must be a number";
-        }
-        if (!LS.isnumber(vy)) {
-            return "scan configuration: 'radiusy' must be a number";
-        }
-        if (!LS.isnumber(vz)) {
-            return "scan configuration: 'radiusz' must be a number";
-        }
+        LS.checknumber(vx, "radiusx");
+        LS.checknumber(vy, "radiusy");
+        LS.checknumber(vz, "radiusz");
         if (have_radius) {
-            return "scan configuration: specified both 'radius' and 'radiusx'";
+            luaL_error(L, "scan configuration: specified both 'radius' and 'radiusx'");
         }
         radius_.x = LS.cknumber(vx);
         radius_.y = LS.cknumber(vy);
         radius_.z = LS.cknumber(vz);
         have_radius = true;
-        parameters += 3;
     }
 
-    LS.rawget(val, config, "shape");
-    if (!LS.isnil(val)) {
-        if (!LS.isstring(val)) {
-            return "scan configuration: 'shape' must be a string";
-        }
+    if (kp.parse(val, "shape")) {
+        LS.checkstring(val, "shape");
         eng::string shape = LS.ckstring(val);
         if (shape == "box") {
             shape_ = BOX;
@@ -919,50 +890,40 @@ eng::string PlaneScan::configure(const LuaStack &LS0, LuaSlot config) {
         } else if (shape == "cylinder") {
             shape_ = CYLINDER;
         } else {
-            return util::ss("scan configuration: unknown shape ", shape);
+            luaL_error(L, "scan configuration: unknown shape %s", shape.c_str());
         }
         have_shape = true;
-        parameters += 1; 
     }
 
-    LS.rawget(val, config, "near");
-    if (!LS.isnil(val)) {
+    if (kp.parse(val, "near")) {
         int64_t id = LS.tanid(val);
         if (id == 0) {
-            return "scan configuration: 'near' must be a tangible";
+            luaL_error(L, "scan configuration: 'near' must be a tangible");
         }
         if (have_center) {
-            return "scan configuration: specified both 'center' and 'near'";
+            luaL_error(L, "scan configuration: specified both 'center' and 'near'");
         }
         if (have_plane) {
-            return "scan configuration: specified both 'plane' and 'near'";
+            luaL_error(L, "scan configuration: specified both 'plane' and 'near'");
         }
         near_ = id;
         have_center = true;
         have_plane = true;
         have_near = true;
-        parameters += 1;
     }
 
-    LS.rawget(val, config, "include");
-    if (!LS.isnil(val)) {
-        if (!LS.isboolean(val)) {
-            return "scan configuration: 'include' must be a boolean";
-        }
+    if (kp.parse(val, "include")) {
+        LS.checkboolean(val, "include");
         if (!have_near) {
-            return "scan configuration: 'include' specified without 'near'";
+            luaL_error(L, "scan configuration: 'include' specified without 'near'");
         }
         include_near_ = LS.ckboolean(val);
-        parameters += 1;
     }
 
-    LS.rawget(val, config, "wholeplane");
-    if (!LS.isnil(val)) {
-        if (!LS.isstring(val)) {
-            return "scan configuration: 'wholeplane' must be a string";
-        }
+    if (kp.parse(val, "wholeplane")) {
+        LS.checkstring(val, "wholeplane");
         if (have_plane || have_center || have_radius || have_shape) {
-            return "scan configuration: do not specify plane, center, shape, or radius with 'wholeplane'";
+            luaL_error(L, "scan configuration: do not specify plane, center, shape, or radius with 'wholeplane'");
         }
         plane_ = LS.ckstring(val);
         set_whole_plane();
@@ -970,26 +931,20 @@ eng::string PlaneScan::configure(const LuaStack &LS0, LuaSlot config) {
         have_center = true;
         have_radius = true;
         have_shape = true;
-        parameters += 1;
-    }
-
-    if (lua_nkeys(LS.state(), config.index()) != parameters) {
-        return "scan configuration: unrecognized parameters in table";
     }
 
     if (!have_plane) {
-        return "scan configuration: did not specify plane";
+        luaL_error(L, "scan configuration: did not specify plane");
     }
     if (!have_radius) {
-        return "scan configuration: did not specify radius";
+        luaL_error(L, "scan configuration: did not specify radius");
     }
     if (!have_center) {
-        return "scan configuration: did not specify center";
+        luaL_error(L, "scan configuration: did not specify center");
     }
     if (!have_shape) {
         shape_ = SPHERE; // default value.
     }
-    return "";
 }
 
 eng::string PlaneScan::debug_string() const {
diff --git a/luprex/core/cpp/table.cpp b/luprex/core/cpp/table.cpp
index 05cb235d..cc722c35 100644
--- a/luprex/core/cpp/table.cpp
+++ b/luprex/core/cpp/table.cpp
@@ -8,8 +8,8 @@
 bool table_equal(LuaStack &LS, LuaSlot t1, LuaSlot t2) {
     lua_State *L = LS.state();
     int top = lua_gettop(L);
-    LS.checktable(t1);
-    LS.checktable(t2);
+    LS.checktable(t1, "table1");
+    LS.checktable(t2, "table2");
     int nkeys1 = lua_nkeys(L, t1.index());
     int nkeys2 = lua_nkeys(L, t2.index());
     if (nkeys1 != nkeys2) return false;
@@ -122,7 +122,7 @@ LuaDefine(table_clear, "table,metaflag", "clear all keys, and optionally the met
     LuaArg tab, clearmeta;
     LuaVar metatable, metafield;
     LuaStack LS(L, tab, clearmeta, metatable, metafield);
-    LS.checktable(tab);
+    LS.checktable(tab, "table");
     if (LS.ckboolean(clearmeta)) {
         LS.getmetatable(metatable, tab);
         if (LS.istable(metatable)) {
@@ -417,9 +417,9 @@ LuaDefine(deque_size, "deque", "return the number of items in the deque") {
     LuaArg deque;
     LuaRet size;
     LuaStack LS(L, deque, size);
-    LS.checktable(deque);
+    LS.checktable(deque, "deque");
     LS.rawget(size, deque, DEQUE_FILL);
-    LS.checknumber(size);
+    LS.checknumber(size, "deque size");
     return LS.result();
 }
 
diff --git a/luprex/core/cpp/world-accessor.cpp b/luprex/core/cpp/world-accessor.cpp
index c9772913..c7413dce 100644
--- a/luprex/core/cpp/world-accessor.cpp
+++ b/luprex/core/cpp/world-accessor.cpp
@@ -138,7 +138,7 @@ LuaDefine(tangible_build, "configtable",
     LuaRet database;
     LuaStack LS(L, config, classname, classtab, database, mt);
     
-    LS.checktable(config);
+    LS.checktable(config, "config");
     // Get the class of the new tangible.
     LS.rawget(classname, config, "class");
     eng::string err = LS.getclass(classtab, classname);
@@ -346,8 +346,13 @@ LuaDefine(tangible_find, "config",
     "|"
     "|   wholeplane: the name of the plane to scan in its entirety"
     "|"
-    "|It is valid to use 0.0 as the radius.  If you do so, then only"
-    "|objects at the exact center you specify will be found."
+    "|It is valid to use 0.0 as a radius.  For example, you could"
+    "|use shape='cylinder' and radiusz=0.0 to scan a flat"
+    "|circular area."
+    "|"
+    "|It is also valid to use math.huge (infinity) as a radius.  For"
+    "|example, you could use shape='cylinder' and radiusz=math.huge"
+    "|to scan an infinitely tall cylinder."
     "|"
     "|If you are making a 2D game, it works fine to set all object Z"
     "|coordinates to zero, and then do searches with centerz=0.0"
@@ -356,11 +361,11 @@ LuaDefine(tangible_find, "config",
     LuaArg config;
     LuaRet result;
     LuaStack LS(L, config, result);
+    LuaKeywordParser kw(LS, config);
     PlaneScan scan;
-    eng::string error = scan.configure(LS, config);
-    if (!error.empty()) {
-        luaL_error(L, "%s", error.c_str());
-    }
+    scan.configure(kw);
+    kw.check_unparsed_keywords();
+
     // When the configure routine sees the 'near' flag, it stores the tangible
     // ID, but not the center and plane, because doing so would require it to
     // know about world models.  We have to handle center and plane for 'near'
diff --git a/luprex/core/cpp/world-difftab.cpp b/luprex/core/cpp/world-difftab.cpp
index 59f0c10f..d7410895 100644
--- a/luprex/core/cpp/world-difftab.cpp
+++ b/luprex/core/cpp/world-difftab.cpp
@@ -463,10 +463,10 @@ LuaDefine(table_diffcompare, "mtnmap,mtab,stnmap,stab", "for unit testing only")
     LuaVar tthread;
     LuaStack MLS(L, mtnmap, mtab, mstnmap, mstab, dbgstring, tthread);
     // Check the arguments.
-    MLS.checktable(mtnmap);
-    MLS.checktable(mstnmap);
-    MLS.checktable(mtab);
-    MLS.checktable(mstab);
+    MLS.checktable(mtnmap, "mtnmap");
+    MLS.checktable(mstnmap, "mstnmap");
+    MLS.checktable(mtab, "mtab");
+    MLS.checktable(mstab, "mstab");
 
     // Create a temporary thread to be the 'synch model'.  We'll use the
     // existing thread as the 'master model'.  Move two tables to the synch thread.
@@ -493,9 +493,9 @@ LuaDefine(table_diffapply, "mtnmap,mtab,mstab", "for unit testing only") {
     LuaVar tthread, tangibles, mntmap, key, val;
     LuaStack MLS(L, mtnmap, mtab, mstab, eql, eqlstr, rtab, tthread, tangibles, mntmap, key, val);
     // Check the arguments.
-    MLS.checktable(mtnmap);
-    MLS.checktable(mtab);
-    MLS.checktable(mstab);
+    MLS.checktable(mtnmap, "mtnmap");
+    MLS.checktable(mtab, "mtab");
+    MLS.checktable(mstab, "mstab");
 
     // Get the tangibles map.
     MLS.rawget(tangibles, LuaRegistry, "tangibles");