diff --git a/ai.c b/ai.c
index 52c607b201..2cb475997b 100644
--- a/ai.c
+++ b/ai.c
@@ -121,7 +121,7 @@ static int AiChooseTrainToBuild(byte railtype, int32 money, byte flag, TileIndex
 {
 	int best_veh_index = -1;
 	byte best_veh_score = 0;
-	int32 r;
+	int32 ret;
 	int i;
 
 	for (i = 0; i < NUM_TRAIN_ENGINES; i++) {
@@ -132,10 +132,8 @@ static int AiChooseTrainToBuild(byte railtype, int32 money, byte flag, TileIndex
 		    || !HASBIT(e->player_avail, _current_player) || e->reliability < 0x8A3D)
 			continue;
 
-		r = DoCommandByTile(tile, i, 0, 0, CMD_BUILD_RAIL_VEHICLE);
-		if (r != CMD_ERROR &&
-					(!(_cmd_build_rail_veh_var1&1) || !(flag&1)) &&
-					r <= money &&
+		ret = DoCommandByTile(tile, i, 0, 0, CMD_BUILD_RAIL_VEHICLE);
+		if (!CmdFailed(ret) && (!(_cmd_build_rail_veh_var1&1) || !(flag&1)) && ret <= money &&
 				_cmd_build_rail_veh_score >= best_veh_score) {
 			best_veh_score = _cmd_build_rail_veh_score;
 			best_veh_index = i;
@@ -149,7 +147,7 @@ static int AiChooseRoadVehToBuild(byte cargo, int32 money, TileIndex tile)
 {
 	int best_veh_index = -1;
 	int32 best_veh_cost = 0;
-	int32 r;
+	int32 ret;
 
 	int i = _cargoc.ai_roadveh_start[cargo];
 	int end = i + _cargoc.ai_roadveh_count[cargo];
@@ -158,9 +156,9 @@ static int AiChooseRoadVehToBuild(byte cargo, int32 money, TileIndex tile)
 		if (!HASBIT(e->player_avail, _current_player) || e->reliability < 0x8A3D)
 			continue;
 
-		r = DoCommandByTile(tile, i, 0, 0, CMD_BUILD_ROAD_VEH);
-		if (r != CMD_ERROR && r <= money && r >= best_veh_cost) {
-			best_veh_cost = r;
+		ret = DoCommandByTile(tile, i, 0, 0, CMD_BUILD_ROAD_VEH);
+		if (!CmdFailed(ret) && ret <= money && ret >= best_veh_cost) {
+			best_veh_cost = ret;
 			best_veh_index = i;
 		}
 	} while (++e, ++i != end);
@@ -172,7 +170,7 @@ static int AiChooseAircraftToBuild(int32 money, byte flag)
 {
 	int best_veh_index = -1;
 	int32 best_veh_cost = 0;
-	int32 r;
+	int32 ret;
 
 	int i = AIRCRAFT_ENGINES_INDEX;
 	int end = i + NUM_AIRCRAFT_ENGINES;
@@ -187,9 +185,9 @@ static int AiChooseAircraftToBuild(int32 money, byte flag)
 			if (i>=253) continue;
 		}
 
-		r = DoCommandByTile(0, i, 0, 0, CMD_BUILD_AIRCRAFT);
-		if (r != CMD_ERROR && r <= money && r >= best_veh_cost) {
-			best_veh_cost = r;
+		ret = DoCommandByTile(0, i, 0, 0, CMD_BUILD_AIRCRAFT);
+		if (!CmdFailed(ret) && ret <= money && ret >= best_veh_cost) {
+			best_veh_cost = ret;
 			best_veh_index = i;
 		}
 	} while (++e, ++i != end);
@@ -272,7 +270,7 @@ static void AiRestoreVehicleOrders(Vehicle *v, BackuppedOrders *bak)
 	int i;
 
 	for (i = 0; bak->order[i].type != OT_NOTHING; i++)
-		if (!DoCommandP(0, v->index + (i << 16), PackOrder(&bak->order[i]), NULL, CMD_INSERT_ORDER | CMD_NO_TEST_IF_IN_NETWORK))
+		if (CmdFailed(DoCommandP(0, v->index + (i << 16), PackOrder(&bak->order[i]), NULL, CMD_INSERT_ORDER | CMD_NO_TEST_IF_IN_NETWORK)))
 			break;
 }
 
@@ -294,8 +292,8 @@ static void AiHandleReplaceTrain(Player *p)
 		BackupVehicleOrders(v, orderbak);
 		tile = v->tile;
 
-		if (DoCommandByTile(0, v->index, 2, DC_EXEC, CMD_SELL_RAIL_WAGON) != CMD_ERROR &&
-			 DoCommandByTile(tile, veh, 0, DC_EXEC, CMD_BUILD_RAIL_VEHICLE) != CMD_ERROR) {
+		if (!CmdFailed(DoCommandByTile(0, v->index, 2, DC_EXEC, CMD_SELL_RAIL_WAGON)) &&
+			  !CmdFailed(DoCommandByTile(tile, veh, 0, DC_EXEC, CMD_BUILD_RAIL_VEHICLE)) ) {
 			veh = _new_train_id;
 			AiRestoreVehicleOrders(GetVehicle(veh), orderbak);
 			DoCommandByTile(0, veh, 0, DC_EXEC, CMD_START_STOP_TRAIN);
@@ -322,8 +320,8 @@ static void AiHandleReplaceRoadVeh(Player *p)
 		BackupVehicleOrders(v, orderbak);
 		tile = v->tile;
 
-		if (DoCommandByTile(0, v->index, 0, DC_EXEC, CMD_SELL_ROAD_VEH) != CMD_ERROR &&
-			 DoCommandByTile(tile, veh, 0, DC_EXEC, CMD_BUILD_ROAD_VEH) != CMD_ERROR) {
+		if (!CmdFailed(DoCommandByTile(0, v->index, 0, DC_EXEC, CMD_SELL_ROAD_VEH)) &&
+			  !CmdFailed(DoCommandByTile(tile, veh, 0, DC_EXEC, CMD_BUILD_ROAD_VEH)) ) {
 			veh = _new_roadveh_id;
 			AiRestoreVehicleOrders(GetVehicle(veh), orderbak);
 			DoCommandByTile(0, veh, 0, DC_EXEC, CMD_START_STOP_ROADVEH);
@@ -350,8 +348,8 @@ static void AiHandleReplaceAircraft(Player *p)
 		BackupVehicleOrders(v, orderbak);
 		tile = v->tile;
 
-		if (DoCommandByTile(0, v->index, 0, DC_EXEC, CMD_SELL_AIRCRAFT) != CMD_ERROR &&
-			 DoCommandByTile(tile, veh, 0, DC_EXEC, CMD_BUILD_AIRCRAFT) != CMD_ERROR) {
+		if (!CmdFailed(DoCommandByTile(0, v->index, 0, DC_EXEC, CMD_SELL_AIRCRAFT)) &&
+			  !CmdFailed(DoCommandByTile(tile, veh, 0, DC_EXEC, CMD_BUILD_AIRCRAFT)) ) {
 			veh = _new_aircraft_id;
 			AiRestoreVehicleOrders(GetVehicle(veh), orderbak);
 			DoCommandByTile(0, veh, 0, DC_EXEC, CMD_START_STOP_AIRCRAFT);
@@ -1567,7 +1565,7 @@ static bool AiCheckTrackResources(TileIndex tile, const AiDefaultBlockData *p, b
 
 static int32 AiDoBuildDefaultRailTrack(TileIndex tile, const AiDefaultBlockData *p, byte flag)
 {
-	int32 r;
+	int32 ret;
 	int32 total_cost = 0;
 	Town *t = NULL;
 	int rating = 0;
@@ -1582,14 +1580,14 @@ static int32 AiDoBuildDefaultRailTrack(TileIndex tile, const AiDefaultBlockData
 		if (p->mode < 2) {
 			if (p->mode == 0) {
 				// Depot
-				r = DoCommandByTile(c, _cur_ai_player->ai.railtype_to_use, p->attr, flag | DC_AUTO | DC_NO_WATER | DC_AI_BUILDING, CMD_BUILD_TRAIN_DEPOT);
+				ret = DoCommandByTile(c, _cur_ai_player->ai.railtype_to_use, p->attr, flag | DC_AUTO | DC_NO_WATER | DC_AI_BUILDING, CMD_BUILD_TRAIN_DEPOT);
 			} else {
 				// Station
-				r = DoCommandByTile(c, (p->attr&1) | (p->attr>>4)<<8 | (p->attr>>1&7)<<16, _cur_ai_player->ai.railtype_to_use, flag | DC_AUTO | DC_NO_WATER | DC_AI_BUILDING, CMD_BUILD_RAILROAD_STATION);
+				ret = DoCommandByTile(c, (p->attr&1) | (p->attr>>4)<<8 | (p->attr>>1&7)<<16, _cur_ai_player->ai.railtype_to_use, flag | DC_AUTO | DC_NO_WATER | DC_AI_BUILDING, CMD_BUILD_RAILROAD_STATION);
 			}
 
-			if (r == CMD_ERROR) return CMD_ERROR;
-			total_cost += r;
+			if (CmdFailed(ret)) return CMD_ERROR;
+			total_cost += ret;
 
 clear_town_stuff:;
 			if (_cleared_town != NULL) {
@@ -1610,36 +1608,35 @@ clear_town_stuff:;
 			for(i=0; i!=6; i++,j>>=1) {
 				if (j&1) {
 					k = i;
-					r = DoCommandByTile(c, _cur_ai_player->ai.railtype_to_use, i, flag | DC_AUTO | DC_NO_WATER, CMD_BUILD_SINGLE_RAIL);
-					if (r == CMD_ERROR) return CMD_ERROR;
-					total_cost += r;
+					ret = DoCommandByTile(c, _cur_ai_player->ai.railtype_to_use, i, flag | DC_AUTO | DC_NO_WATER, CMD_BUILD_SINGLE_RAIL);
+					if (CmdFailed(ret)) return CMD_ERROR;
+					total_cost += ret;
 				}
 			}
 
 			/* signals too? */
 			if (j&3) {
 				// Can't build signals on a road.
-				if (IsTileType(c, MP_STREET))
-					return CMD_ERROR;
+				if (IsTileType(c, MP_STREET)) return CMD_ERROR;
 
 				if (flag & DC_EXEC) {
 					j = 4 - j;
 					do {
-						r = DoCommandByTile(c, k, 0, flag, CMD_BUILD_SIGNALS);
+						ret = DoCommandByTile(c, k, 0, flag, CMD_BUILD_SIGNALS);
 					} while (--j);
 				} else {
-					r = _price.build_signals;
+					ret = _price.build_signals;
 				}
-				if (r == CMD_ERROR) return CMD_ERROR;
-				total_cost += r;
+				if (CmdFailed(ret)) return CMD_ERROR;
+				total_cost += ret;
 			}
 		} else if (p->mode == 3) {
 			//Clear stuff and then build single rail.
 			if (GetTileSlope(c,NULL) != 0)
 				return CMD_ERROR;
-			r = DoCommandByTile(c, 0, 0, flag | DC_AUTO | DC_NO_WATER | DC_AI_BUILDING, CMD_LANDSCAPE_CLEAR);
-			if (r == CMD_ERROR) return CMD_ERROR;
-			total_cost += r + _price.build_rail;
+			ret = DoCommandByTile(c, 0, 0, flag | DC_AUTO | DC_NO_WATER | DC_AI_BUILDING, CMD_LANDSCAPE_CLEAR);
+			if (CmdFailed(ret)) return CMD_ERROR;
+			total_cost += ret + _price.build_rail;
 
 			if (flag & DC_EXEC) {
 				DoCommandByTile(c, _cur_ai_player->ai.railtype_to_use, p->attr&1, flag | DC_AUTO | DC_NO_WATER | DC_AI_BUILDING, CMD_BUILD_SINGLE_RAIL);
@@ -1976,8 +1973,8 @@ static inline void AiCheckBuildRailBridgeHere(AiRailFinder *arf, TileIndex tile,
 		}
 
 		// Is building a (rail)bridge possible at this place (type doesn't matter)?
-		if (DoCommandByTile(tile_new, tile, 0 | arf->player->ai.railtype_to_use << 8,
-			DC_AUTO, CMD_BUILD_BRIDGE) == CMD_ERROR)
+		if (CmdFailed(DoCommandByTile(tile_new, tile, 0 | arf->player->ai.railtype_to_use << 8,
+			DC_AUTO, CMD_BUILD_BRIDGE)) )
 				return;
 		AiBuildRailRecursive(arf, tile_new, dir2);
 
@@ -1996,7 +1993,7 @@ static inline void AiCheckBuildRailTunnelHere(AiRailFinder *arf, TileIndex tile,
 	if (arf->ti.tileh == _dir_table_2[p[0]&3] && arf->ti.z!=0) {
 		int32 cost = DoCommandByTile(tile, arf->player->ai.railtype_to_use, 0, DC_AUTO, CMD_BUILD_TUNNEL);
 
-		if (cost != CMD_ERROR && cost <= (arf->player->player_money>>4)) {
+		if (!CmdFailed(cost) && cost <= (arf->player->player_money>>4)) {
 			AiBuildRailRecursive(arf, _build_tunnel_endtile, p[0]&3);
 			if (arf->depth == 1) {
 				AiCheckRailPathBetter(arf, p);
@@ -2054,7 +2051,7 @@ static void AiBuildRailRecursive(AiRailFinder *arf, TileIndex tile, int dir)
 		do {
 			// Make sure the tile is not in the list of banned tiles and that a rail can be built here.
 			if (!AiIsTileBanned(arf->player, tile, p[0]) &&
-					DoCommandByTile(tile, arf->player->ai.railtype_to_use, p[0], DC_AUTO | DC_NO_WATER | DC_NO_RAIL_OVERLAP, CMD_BUILD_SINGLE_RAIL) != CMD_ERROR) {
+					!CmdFailed(DoCommandByTile(tile, arf->player->ai.railtype_to_use, p[0], DC_AUTO | DC_NO_WATER | DC_NO_RAIL_OVERLAP, CMD_BUILD_SINGLE_RAIL))) {
 				AiBuildRailRecursive(arf, tile, p[1]);
 			}
 
@@ -2142,7 +2139,7 @@ static void AiBuildRailConstruct(Player *p)
 		for (i = MAX_BRIDGES - 1; i != 0; i--) {
 			if (CheckBridge_Stuff(i, bridge_len)) {
 				int32 cost = DoCommandByTile(arf.bridge_end_tile, p->ai.cur_tile_a, i | (p->ai.railtype_to_use << 8), DC_AUTO, CMD_BUILD_BRIDGE);
-				if (cost != CMD_ERROR && cost < (p->player_money >> 5))
+				if (!CmdFailed(cost) && cost < (p->player_money >> 5))
 					break;
 			}
 		}
@@ -2183,7 +2180,7 @@ static bool AiRemoveTileAndGoForward(Player *p)
 	if (IsTileType(tile, MP_TUNNELBRIDGE)) {
 		if (!(_map5[tile] & 0x80)) {
 			// Clear the tunnel and continue at the other side of it.
-			if (DoCommandByTile(tile, 0, 0, DC_EXEC, CMD_LANDSCAPE_CLEAR) == CMD_ERROR)
+			if (CmdFailed(DoCommandByTile(tile, 0, 0, DC_EXEC, CMD_LANDSCAPE_CLEAR)) )
 				return false;
 			p->ai.cur_tile_a = TILE_MASK(_build_tunnel_endtile - TileOffsByDir(p->ai.cur_dir_a));
 			return true;
@@ -2204,7 +2201,7 @@ static bool AiRemoveTileAndGoForward(Player *p)
 
 			tilenew = TILE_MASK(tile - offs);
 			// And clear the bridge.
-			if (DoCommandByTile(tile, 0, 0, DC_EXEC, CMD_LANDSCAPE_CLEAR) == CMD_ERROR)
+			if (CmdFailed(DoCommandByTile(tile, 0, 0, DC_EXEC, CMD_LANDSCAPE_CLEAR)) )
 				return false;
 			p->ai.cur_tile_a = tilenew;
 			return true;
@@ -2226,7 +2223,7 @@ static bool AiRemoveTileAndGoForward(Player *p)
 	}
 
 	// And also remove the rail.
-	if (DoCommandByTile(tile, 0, bit, DC_EXEC, CMD_REMOVE_SINGLE_RAIL) == CMD_ERROR)
+	if (CmdFailed(DoCommandByTile(tile, 0, bit, DC_EXEC, CMD_REMOVE_SINGLE_RAIL)) )
 		return false;
 
 	// Find the direction at the other edge of the rail.
@@ -2382,7 +2379,7 @@ static void AiStateBuildRailVeh(Player *p)
 		if (p->ai.wagon_list[i] == INVALID_VEHICLE) {
 			veh = _cargoc.ai_railwagon[p->ai.railtype_to_use][cargo];
 			cost = DoCommandByTile(tile, veh, 0, DC_EXEC, CMD_BUILD_RAIL_VEHICLE);
-			if (cost == CMD_ERROR) goto handle_nocash;
+			if (CmdFailed(cost)) goto handle_nocash;
 			p->ai.wagon_list[i] = _new_wagon_id;
 			p->ai.wagon_list[i+1] = INVALID_VEHICLE;
 			return;
@@ -2401,7 +2398,7 @@ handle_nocash:
 		if (++p->ai.state_counter == 1000) {
 			for(i=0; p->ai.wagon_list[i] != INVALID_VEHICLE; i++) {
 				cost = DoCommandByTile(tile, p->ai.wagon_list[i], 0, DC_EXEC, CMD_SELL_RAIL_WAGON);
-				assert(cost != CMD_ERROR);
+				assert(!CmdFailed(cost));
 			}
 			p->ai.state =	AIS_0;
 		}
@@ -2410,7 +2407,7 @@ handle_nocash:
 
 	// Try to build the locomotive
 	cost = DoCommandByTile(tile, veh, 0, DC_EXEC, CMD_BUILD_RAIL_VEHICLE);
-	assert(cost != CMD_ERROR);
+	assert(!CmdFailed(cost));
 	loco_id = _new_train_id;
 
 	// Sell a vehicle if the train is double headed.
@@ -2531,7 +2528,7 @@ static int AiFindBestDefaultRoadBlock(TileIndex tile, byte direction, byte cargo
 
 static int32 AiDoBuildDefaultRoadBlock(TileIndex tile, const AiDefaultBlockData *p, byte flag)
 {
-	int32 r;
+	int32 ret;
 	int32 total_cost = 0;
 	Town *t = NULL;
 	int rating = 0;
@@ -2553,30 +2550,29 @@ static int32 AiDoBuildDefaultRoadBlock(TileIndex tile, const AiDefaultBlockData
 					continue;
 			}
 
-			r = DoCommandByTile(c, p->attr, 0, flag | DC_AUTO | DC_NO_WATER, CMD_BUILD_ROAD);
-			if (r == CMD_ERROR)
-				return CMD_ERROR;
-			total_cost += r;
+			ret = DoCommandByTile(c, p->attr, 0, flag | DC_AUTO | DC_NO_WATER, CMD_BUILD_ROAD);
+			if (CmdFailed(ret)) return CMD_ERROR;
+			total_cost += ret;
 
 			continue;
 		}
 
 		if (p->mode == 0) {
 			// Depot
-			r = DoCommandByTile(c, p->attr, 0, flag | DC_AUTO | DC_NO_WATER | DC_AI_BUILDING, CMD_BUILD_ROAD_DEPOT);
+			ret = DoCommandByTile(c, p->attr, 0, flag | DC_AUTO | DC_NO_WATER | DC_AI_BUILDING, CMD_BUILD_ROAD_DEPOT);
 			goto clear_town_stuff;
 		} else if (p->mode == 1) {
 			if (_want_road_truck_station) {
 				// Truck station
-				r = DoCommandByTile(c, p->attr, RS_TRUCK, flag | DC_AUTO | DC_NO_WATER | DC_AI_BUILDING, CMD_BUILD_ROAD_STOP);
+				ret = DoCommandByTile(c, p->attr, RS_TRUCK, flag | DC_AUTO | DC_NO_WATER | DC_AI_BUILDING, CMD_BUILD_ROAD_STOP);
 			} else {
 				// Bus station
-				r = DoCommandByTile(c, p->attr, RS_BUS, flag | DC_AUTO | DC_NO_WATER | DC_AI_BUILDING, CMD_BUILD_ROAD_STOP);
+				ret = DoCommandByTile(c, p->attr, RS_BUS, flag | DC_AUTO | DC_NO_WATER | DC_AI_BUILDING, CMD_BUILD_ROAD_STOP);
 			}
 clear_town_stuff:;
 
-			if (r == CMD_ERROR) return CMD_ERROR;
-			total_cost += r;
+			if (CmdFailed(ret)) return CMD_ERROR;
+			total_cost += ret;
 
 			if (_cleared_town != NULL) {
 				if (t != NULL && t != _cleared_town)
@@ -2592,8 +2588,8 @@ clear_town_stuff:;
 				return CMD_ERROR;
 
 			if (!(IsTileType(c, MP_STREET) && (_map5[c] & 0xF0) == 0)) {
-				r = DoCommandByTile(c, 0, 0, flag | DC_AUTO | DC_NO_WATER | DC_AI_BUILDING, CMD_LANDSCAPE_CLEAR);
-				if (r == CMD_ERROR) return CMD_ERROR;
+				ret = DoCommandByTile(c, 0, 0, flag | DC_AUTO | DC_NO_WATER | DC_AI_BUILDING, CMD_LANDSCAPE_CLEAR);
+				if (CmdFailed(ret)) return CMD_ERROR;
 			}
 
 		}
@@ -2836,7 +2832,7 @@ static bool AiBuildRoadHelper(uint tile, int flags, int type)
 		1+2,
 		8+4,
 	};
-	return DoCommandByTile(tile, _road_bits[type], 0, flags, CMD_BUILD_ROAD) != CMD_ERROR;
+	return !CmdFailed(DoCommandByTile(tile, _road_bits[type], 0, flags, CMD_BUILD_ROAD));
 }
 
 static inline void AiCheckBuildRoadBridgeHere(AiRoadFinder *arf, TileIndex tile, const byte *p)
@@ -2867,7 +2863,7 @@ static inline void AiCheckBuildRoadBridgeHere(AiRoadFinder *arf, TileIndex tile,
 		}
 
 		// Is building a (rail)bridge possible at this place (type doesn't matter)?
-		if (DoCommandByTile(tile_new, tile, 0x8000, DC_AUTO, CMD_BUILD_BRIDGE) == CMD_ERROR)
+		if (CmdFailed(DoCommandByTile(tile_new, tile, 0x8000, DC_AUTO, CMD_BUILD_BRIDGE)))
 			return;
 		AiBuildRoadRecursive(arf, tile_new, dir2);
 
@@ -2886,7 +2882,7 @@ static inline void AiCheckBuildRoadTunnelHere(AiRoadFinder *arf, TileIndex tile,
 	if (arf->ti.tileh == _dir_table_2[p[0]&3] && arf->ti.z!=0) {
 		int32 cost = DoCommandByTile(tile, 0x200, 0, DC_AUTO, CMD_BUILD_TUNNEL);
 
-		if (cost != CMD_ERROR && cost <= (arf->player->player_money>>4)) {
+		if (!CmdFailed(cost) && cost <= (arf->player->player_money>>4)) {
 			AiBuildRoadRecursive(arf, _build_tunnel_endtile, p[0]&3);
 			if (arf->depth == 1) {
 				AiCheckRoadPathBetter(arf, p);
@@ -3026,7 +3022,7 @@ do_some_terraform:
 		for(i = 10; i != 0; i--) {
 			if (CheckBridge_Stuff(i, bridge_len)) {
 				int32 cost = DoCommandByTile(tile, p->ai.cur_tile_a, i + (0x80 << 8), DC_AUTO, CMD_BUILD_BRIDGE);
-				if (cost != CMD_ERROR && cost < (p->player_money >> 5))
+				if (!CmdFailed(cost) && cost < (p->player_money >> 5))
 					break;
 			}
 		}
@@ -3168,7 +3164,6 @@ static void AiStateBuildRoadVehicles(Player *p)
 	const AiDefaultBlockData *ptr;
 	uint tile,loco_id;
 	int veh, i;
-	int32 cost;
 
 	ptr = _road_default_block_data[p->ai.src.cur_building_rule]->data;
 	for(;ptr->mode != 0;ptr++) {}
@@ -3180,9 +3175,7 @@ static void AiStateBuildRoadVehicles(Player *p)
 		return;
 	}
 
-	cost = DoCommandByTile(tile, veh, 0, DC_EXEC, CMD_BUILD_ROAD_VEH);
-	if (cost == CMD_ERROR)
-		return;
+	if (CmdFailed(DoCommandByTile(tile, veh, 0, DC_EXEC, CMD_BUILD_ROAD_VEH))) return;
 
 	loco_id = _new_roadveh_id;
 
@@ -3336,15 +3329,14 @@ static void AiStateAirportStuff(Player *p)
 
 static int32 AiDoBuildDefaultAirportBlock(TileIndex tile, const AiDefaultBlockData *p, byte flag)
 {
-	int32 total_cost = 0, r;
+	int32 total_cost = 0, ret;
 
 	for(;p->mode == 0;p++) {
 		if (!HASBIT(_avail_aircraft, p->attr))
 			return CMD_ERROR;
-		r = DoCommandByTile(TILE_MASK(tile + ToTileIndexDiff(p->tileoffs)), p->attr,0,flag | DC_AUTO | DC_NO_WATER,CMD_BUILD_AIRPORT);
-		if (r == CMD_ERROR)
-			return CMD_ERROR;
-		total_cost += r;
+		ret = DoCommandByTile(TILE_MASK(tile + ToTileIndexDiff(p->tileoffs)), p->attr,0,flag | DC_AUTO | DC_NO_WATER,CMD_BUILD_AIRPORT);
+		if (CmdFailed(ret)) return CMD_ERROR;
+		total_cost += ret;
 	}
 
 	return total_cost;
@@ -3489,7 +3481,6 @@ static void AiStateBuildAircraftVehicles(Player *p)
 	const AiDefaultBlockData *ptr;
 	uint tile;
 	int veh;
-	int32 cost;
 	int i;
 	uint loco_id;
 
@@ -3503,9 +3494,7 @@ static void AiStateBuildAircraftVehicles(Player *p)
 		return;
 	}
 
-	cost = DoCommandByTile(tile, veh, 0, DC_EXEC, CMD_BUILD_AIRCRAFT);
-	if (cost == CMD_ERROR)
-		return;
+	if (CmdFailed(DoCommandByTile(tile, veh, 0, DC_EXEC, CMD_BUILD_AIRCRAFT))) return;
 	loco_id = _new_aircraft_id;
 
 	for(i=0; p->ai.order_list_blocks[i] != 0xFF; i++) {
diff --git a/ai_build.c b/ai_build.c
index 55fab3fbc7..1328bd535a 100644
--- a/ai_build.c
+++ b/ai_build.c
@@ -12,7 +12,7 @@
 //  Params:
 //    tile : tile where HQ is going to be build
 bool AiNew_Build_CompanyHQ(Player *p, uint tile) {
-	if (DoCommandByTile(tile, 0, 0, DC_AUTO | DC_NO_WATER, CMD_BUILD_COMPANY_HQ) == CMD_ERROR)
+	if (CmdFailed(DoCommandByTile(tile, 0, 0, DC_AUTO | DC_NO_WATER, CMD_BUILD_COMPANY_HQ)))
 		return false;
 	DoCommandByTile(tile, 0, 0, DC_EXEC | DC_AUTO | DC_NO_WATER, CMD_BUILD_COMPANY_HQ);
 	return true;
@@ -29,10 +29,11 @@ bool AiNew_Build_CompanyHQ(Player *p, uint tile) {
 int AiNew_Build_Station(Player *p, byte type, uint tile, byte length, byte numtracks, byte direction, byte flag) {
 	if (type == AI_TRAIN)
 		return DoCommandByTile(tile, direction + (numtracks << 8) + (length << 16), 0, flag | DC_AUTO | DC_NO_WATER, CMD_BUILD_RAILROAD_STATION);
-	else if (type == AI_BUS)
+
+	if (type == AI_BUS)
 		return DoCommandByTile(tile, direction, RS_BUS, flag | DC_AUTO | DC_NO_WATER, CMD_BUILD_ROAD_STOP);
-	else
-		return DoCommandByTile(tile, direction, RS_TRUCK, flag | DC_AUTO | DC_NO_WATER, CMD_BUILD_ROAD_STOP);
+
+	return DoCommandByTile(tile, direction, RS_TRUCK, flag | DC_AUTO | DC_NO_WATER, CMD_BUILD_ROAD_STOP);
 }
 
 // Builds a brdige. The second best out of the ones available for this player
@@ -61,8 +62,8 @@ int AiNew_Build_Bridge(Player *p, uint tile_a, uint tile_b, byte flag) {
 	// Now, simply, build the bridge!
 	if (p->ainew.tbt == AI_TRAIN)
 		return DoCommandByTile(tile_a, tile_b, (0<<8) + type2, flag | DC_AUTO, CMD_BUILD_BRIDGE);
-	else
-		return DoCommandByTile(tile_a, tile_b, (0x80 << 8) + type2, flag | DC_AUTO, CMD_BUILD_BRIDGE);
+
+	return DoCommandByTile(tile_a, tile_b, (0x80 << 8) + type2, flag | DC_AUTO, CMD_BUILD_BRIDGE);
 }
 
 
@@ -97,7 +98,7 @@ int AiNew_Build_RoutePart(Player *p, Ai_PathFinderInfo *PathFinderInfo, byte fla
      		cost += DoCommandByTile(route[part], 0, 0, flag, CMD_BUILD_TUNNEL);
      		PathFinderInfo->position++;
      		// TODO: problems!
-     		if (cost == CMD_ERROR) {
+     		if (CmdFailed(cost)) {
      			DEBUG(ai,0)("[AiNew - BuildPath] We have a serious problem: tunnel could not be build!");
 				return 0;
      		}
@@ -108,7 +109,7 @@ int AiNew_Build_RoutePart(Player *p, Ai_PathFinderInfo *PathFinderInfo, byte fla
      		cost += AiNew_Build_Bridge(p, route[part], route[part-1], flag);
      		PathFinderInfo->position++;
      		// TODO: problems!
-     		if (cost == CMD_ERROR) {
+     		if (CmdFailed(cost)) {
      			DEBUG(ai,0)("[AiNew - BuildPath] We have a serious problem: bridge could not be build!");
 				return 0;
      		}
@@ -126,7 +127,7 @@ int AiNew_Build_RoutePart(Player *p, Ai_PathFinderInfo *PathFinderInfo, byte fla
 	     		old_dir = dir;
 	     		// Build the tile
 	     		res = DoCommandByTile(route[part], 0, dir, flag, CMD_BUILD_SINGLE_RAIL);
-	     		if (res == CMD_ERROR) {
+	     		if (CmdFailed(res)) {
 	     			// Problem.. let's just abort it all!
 	     			p->ainew.state = AI_STATE_NOTHING;
 	     			return 0;
@@ -147,7 +148,7 @@ int AiNew_Build_RoutePart(Player *p, Ai_PathFinderInfo *PathFinderInfo, byte fla
      		cost += DoCommandByTile(route[part], 0x200, 0, flag, CMD_BUILD_TUNNEL);
      		PathFinderInfo->position++;
      		// TODO: problems!
-     		if (cost == CMD_ERROR) {
+     		if (CmdFailed(cost)) {
      			DEBUG(ai,0)("[AiNew - BuildPath] We have a serious problem: tunnel could not be build!");
 				return 0;
      		}
@@ -158,7 +159,7 @@ int AiNew_Build_RoutePart(Player *p, Ai_PathFinderInfo *PathFinderInfo, byte fla
      		cost += AiNew_Build_Bridge(p, route[part], route[part+1], flag);
      		PathFinderInfo->position++;
      		// TODO: problems!
-     		if (cost == CMD_ERROR) {
+     		if (CmdFailed(cost)) {
      			DEBUG(ai,0)("[AiNew - BuildPath] We have a serious problem: bridge could not be build!");
 				return 0;
      		}
@@ -181,15 +182,14 @@ int AiNew_Build_RoutePart(Player *p, Ai_PathFinderInfo *PathFinderInfo, byte fla
 	     			// Build the tile
 	     			res = DoCommandByTile(route[part], dir, 0, flag | DC_NO_WATER, CMD_BUILD_ROAD);
 	     			// Currently, we ignore CMD_ERRORs!
-	     			if (res == CMD_ERROR && flag == DC_EXEC && !IsTileType(route[part], MP_STREET) && !EnsureNoVehicle(route[part])) {
-     					// Problem.. let's just abort it all!
-     					DEBUG(ai,0)("Darn, the route could not be builded.. aborting!");
-    	     			p->ainew.state = AI_STATE_NOTHING;
-    	     			return 0;
-    	     		} else {
-    	     			if (res != CMD_ERROR)
-    	     				cost += res;
-    	     		}
+						if (CmdFailed(res) && flag == DC_EXEC && !IsTileType(route[part], MP_STREET) && !EnsureNoVehicle(route[part])) {
+							// Problem.. let's just abort it all!
+							DEBUG(ai,0)("Darn, the route could not be builded.. aborting!");
+							p->ainew.state = AI_STATE_NOTHING;
+							return 0;
+						}
+
+    	     	if (!CmdFailed(res)) cost += res;
 		     	}
 	     		// Go to the next tile
 	     		part++;
@@ -213,7 +213,7 @@ int AiNew_PickVehicle(Player *p) {
         // Not supported yet
         return -1;
     } else {
-        int start, count, i, r = CMD_ERROR;
+        int start, count, i, ret = CMD_ERROR;
         start = _cargoc.ai_roadveh_start[p->ainew.cargo];
         count = _cargoc.ai_roadveh_count[p->ainew.cargo];
 
@@ -223,11 +223,11 @@ int AiNew_PickVehicle(Player *p) {
         	// Also, check if the reliability of the vehicle is above the AI_VEHICLE_MIN_RELIABILTY
         	if (!HASBIT(_engines[i].player_avail, _current_player) || _engines[i].reliability * 100 < AI_VEHICLE_MIN_RELIABILTY << 16) continue;
         	// Can we build it?
-        	r = DoCommandByTile(0, i, 0, DC_QUERY_COST, CMD_BUILD_ROAD_VEH);
-        	if (r != CMD_ERROR) break;
+        	ret = DoCommandByTile(0, i, 0, DC_QUERY_COST, CMD_BUILD_ROAD_VEH);
+        	if (!CmdFailed(ret)) break;
        	}
        	// We did not find a vehicle :(
-       	if (r == CMD_ERROR) { return -1; }
+       	if (CmdFailed(ret)) { return -1; }
        	return i;
     }
 }
@@ -237,25 +237,24 @@ int AiNew_Build_Vehicle(Player *p, uint tile, byte flag) {
 	int i = AiNew_PickVehicle(p);
 	if (i == -1) return CMD_ERROR;
 
-	if (p->ainew.tbt == AI_TRAIN) {
+	if (p->ainew.tbt == AI_TRAIN)
 		return CMD_ERROR;
-	} else {
-		return DoCommandByTile(tile, i, 0, flag, CMD_BUILD_ROAD_VEH);
-	}
+
+	return DoCommandByTile(tile, i, 0, flag, CMD_BUILD_ROAD_VEH);
 }
 
-int AiNew_Build_Depot(Player *p, uint tile, byte direction, byte flag) {
+int AiNew_Build_Depot(Player *p, uint tile, byte direction, byte flag)
+{
 	static const byte _roadbits_by_dir[4] = {2,1,8,4};
-	int r, r2;
-    if (p->ainew.tbt == AI_TRAIN) {
+	int ret, ret2;
+    if (p->ainew.tbt == AI_TRAIN)
     	return DoCommandByTile(tile, 0, direction, flag | DC_AUTO | DC_NO_WATER, CMD_BUILD_TRAIN_DEPOT);
-    } else {
-    	r = DoCommandByTile(tile, direction, 0, flag | DC_AUTO | DC_NO_WATER, CMD_BUILD_ROAD_DEPOT);
-    	if (r == CMD_ERROR) return r;
-    	// Try to build the road from the depot
-    	r2 = DoCommandByTile(tile + TileOffsByDir(direction), _roadbits_by_dir[direction], 0, flag | DC_AUTO | DC_NO_WATER, CMD_BUILD_ROAD);
-    	// If it fails, ignore it..
-    	if (r2 == CMD_ERROR) return r;
-    	return r + r2;
-    }
+
+		ret = DoCommandByTile(tile, direction, 0, flag | DC_AUTO | DC_NO_WATER, CMD_BUILD_ROAD_DEPOT);
+		if (CmdFailed(ret)) return ret;
+		// Try to build the road from the depot
+		ret2 = DoCommandByTile(tile + TileOffsByDir(direction), _roadbits_by_dir[direction], 0, flag | DC_AUTO | DC_NO_WATER, CMD_BUILD_ROAD);
+		// If it fails, ignore it..
+		if (CmdFailed(ret2)) return ret;
+		return ret + ret2;
 }
diff --git a/ai_new.c b/ai_new.c
index d52920556c..3ca443f329 100644
--- a/ai_new.c
+++ b/ai_new.c
@@ -994,7 +994,7 @@ static void AiNew_State_BuildPath(Player *p) {
         	// We don't want that, so try building some road left or right of the station
         	int dir1, dir2, dir3;
         	TileIndex tile;
-        	int i, r;
+        	int i, ret;
         	for (i=0;i<2;i++) {
             	if (i == 0) {
             		tile = p->ainew.from_tile + TileOffsByDir(p->ainew.from_direction);
@@ -1012,36 +1012,36 @@ static void AiNew_State_BuildPath(Player *p) {
             		dir3 = p->ainew.to_direction;
             	}
 
-            	r = DoCommandByTile(tile, _roadbits_by_dir[dir1], 0, DC_EXEC | DC_NO_WATER, CMD_BUILD_ROAD);
-				if (r != CMD_ERROR) {
+            	ret = DoCommandByTile(tile, _roadbits_by_dir[dir1], 0, DC_EXEC | DC_NO_WATER, CMD_BUILD_ROAD);
+				if (!CmdFailed(ret)) {
 					dir1 = TileOffsByDir(dir1);
 					if (IsTileType(tile + dir1, MP_CLEAR) || IsTileType(tile + dir1, MP_TREES)) {
-						r = DoCommandByTile(tile+dir1, AiNew_GetRoadDirection(tile, tile+dir1, tile+dir1+dir1), 0, DC_EXEC | DC_NO_WATER, CMD_BUILD_ROAD);
-						if (r != CMD_ERROR) {
+						ret = DoCommandByTile(tile+dir1, AiNew_GetRoadDirection(tile, tile+dir1, tile+dir1+dir1), 0, DC_EXEC | DC_NO_WATER, CMD_BUILD_ROAD);
+						if (!CmdFailed(ret)) {
 							if (IsTileType(tile + dir1 + dir1, MP_CLEAR) || IsTileType(tile + dir1 + dir1, MP_TREES))
 								DoCommandByTile(tile+dir1+dir1, AiNew_GetRoadDirection(tile+dir1, tile+dir1+dir1, tile+dir1+dir1+dir1), 0, DC_EXEC | DC_NO_WATER, CMD_BUILD_ROAD);
 						}
 					}
 				}
 
-				r = DoCommandByTile(tile, _roadbits_by_dir[dir2], 0, DC_EXEC | DC_NO_WATER, CMD_BUILD_ROAD);
-				if (r != CMD_ERROR) {
+				ret = DoCommandByTile(tile, _roadbits_by_dir[dir2], 0, DC_EXEC | DC_NO_WATER, CMD_BUILD_ROAD);
+				if (!CmdFailed(ret)) {
 					dir2 = TileOffsByDir(dir2);
 					if (IsTileType(tile + dir2, MP_CLEAR) || IsTileType(tile + dir2, MP_TREES)) {
-						r = DoCommandByTile(tile+dir2, AiNew_GetRoadDirection(tile, tile+dir2, tile+dir2+dir2), 0, DC_EXEC | DC_NO_WATER, CMD_BUILD_ROAD);
-						if (r != CMD_ERROR) {
+						ret = DoCommandByTile(tile+dir2, AiNew_GetRoadDirection(tile, tile+dir2, tile+dir2+dir2), 0, DC_EXEC | DC_NO_WATER, CMD_BUILD_ROAD);
+						if (!CmdFailed(ret)) {
 							if (IsTileType(tile + dir2 + dir2, MP_CLEAR) || IsTileType(tile + dir2 + dir2, MP_TREES))
 								DoCommandByTile(tile+dir2+dir2, AiNew_GetRoadDirection(tile+dir2, tile+dir2+dir2, tile+dir2+dir2+dir2), 0, DC_EXEC | DC_NO_WATER, CMD_BUILD_ROAD);
 						}
 					}
 				}
 
-				r = DoCommandByTile(tile, _roadbits_by_dir[dir3^2], 0, DC_EXEC | DC_NO_WATER, CMD_BUILD_ROAD);
-				if (r != CMD_ERROR) {
+				ret = DoCommandByTile(tile, _roadbits_by_dir[dir3^2], 0, DC_EXEC | DC_NO_WATER, CMD_BUILD_ROAD);
+				if (!CmdFailed(ret)) {
 					dir3 = TileOffsByDir(dir3);
 					if (IsTileType(tile + dir3, MP_CLEAR) || IsTileType(tile + dir3, MP_TREES)) {
-						r = DoCommandByTile(tile+dir3, AiNew_GetRoadDirection(tile, tile+dir3, tile+dir3+dir3), 0, DC_EXEC | DC_NO_WATER, CMD_BUILD_ROAD);
-						if (r != CMD_ERROR) {
+						ret = DoCommandByTile(tile+dir3, AiNew_GetRoadDirection(tile, tile+dir3, tile+dir3+dir3), 0, DC_EXEC | DC_NO_WATER, CMD_BUILD_ROAD);
+						if (!CmdFailed(ret)) {
 							if (IsTileType(tile + dir3 + dir3, MP_CLEAR) || IsTileType(tile + dir3 + dir3, MP_TREES))
 								DoCommandByTile(tile+dir3+dir3, AiNew_GetRoadDirection(tile+dir3, tile+dir3+dir3, tile+dir3+dir3+dir3), 0, DC_EXEC | DC_NO_WATER, CMD_BUILD_ROAD);
 						}
@@ -1225,11 +1225,11 @@ static void AiNew_CheckVehicle(Player *p, Vehicle *v) {
 
 			if (!AiNew_SetSpecialVehicleFlag(p, v, AI_VEHICLEFLAG_SELL)) return;
 			{
-				int res = 0;
+				int ret = 0;
 				if (v->type == VEH_Road)
-					res = DoCommandByTile(0, v->index, 0, DC_EXEC, CMD_SEND_ROADVEH_TO_DEPOT);
+					ret = DoCommandByTile(0, v->index, 0, DC_EXEC, CMD_SEND_ROADVEH_TO_DEPOT);
 				// This means we can not find a depot :s
-//				if (res == CMD_ERROR)
+//				if (CmdFailed(ret))
 			}
 		}
 	}
diff --git a/ai_pathfinder.c b/ai_pathfinder.c
index 1d17017b6e..4b6e15d202 100644
--- a/ai_pathfinder.c
+++ b/ai_pathfinder.c
@@ -13,22 +13,21 @@
 // TODO: make it train compatible
 static bool TestCanBuildStationHere(uint tile, byte dir)
 {
-    Player *p = DEREF_PLAYER(_current_player);
-    if (dir == TEST_STATION_NO_DIR) {
-        // TODO: currently we only allow spots that can be access from al 4 directions...
-        //  should be fixed!!!
-        for (dir=0;dir<4;dir++) {
-        	int res = AiNew_Build_Station(p, p->ainew.tbt, tile, 1, 1, dir, DC_QUERY_COST);
-            if (res != CMD_ERROR)
-            	return true;
-        }
-        return false;
-    } else {
-       	int res = AiNew_Build_Station(p, p->ainew.tbt, tile, 1, 1, dir, DC_QUERY_COST);
-        if (res == CMD_ERROR)
-        	return false;
-    }
-    return true;
+	Player *p = DEREF_PLAYER(_current_player);
+
+	if (dir == TEST_STATION_NO_DIR) {
+		int32 ret;
+		// TODO: currently we only allow spots that can be access from al 4 directions...
+		//  should be fixed!!!
+		for (dir=0;dir<4;dir++) {
+			ret = AiNew_Build_Station(p, p->ainew.tbt, tile, 1, 1, dir, DC_QUERY_COST);
+			if (!CmdFailed(ret)) return true;
+		}
+		return false;
+	}
+
+	// return true if command succeeded, so the inverse of CmdFailed()
+	return !CmdFailed(AiNew_Build_Station(p, p->ainew.tbt, tile, 1, 1, dir, DC_QUERY_COST));
 }
 
 
@@ -191,7 +190,7 @@ static void AyStar_AiPathFinder_FoundEndNode(AyStar *aystar, OpenListNode *curre
 // What tiles are around us.
 static void AyStar_AiPathFinder_GetNeighbours(AyStar *aystar, OpenListNode *current) {
 		uint i;
-		int r;
+		int ret;
 		int dir;
 
    	Ai_PathFinderInfo *PathFinderInfo = (Ai_PathFinderInfo*)aystar->user_target;
@@ -248,8 +247,8 @@ static void AyStar_AiPathFinder_GetNeighbours(AyStar *aystar, OpenListNode *curr
        			if (PathFinderInfo->rail_or_road) {
        				// Rail check
        				dir = AiNew_GetRailDirection(current->path.parent->node.tile, current->path.node.tile, current->path.node.tile + TileOffsByDir(i));
-       				r = DoCommandByTile(current->path.node.tile, 0, dir, DC_AUTO | DC_NO_WATER, CMD_BUILD_SINGLE_RAIL);
-       				if (r == CMD_ERROR) continue;
+       				ret = DoCommandByTile(current->path.node.tile, 0, dir, DC_AUTO | DC_NO_WATER, CMD_BUILD_SINGLE_RAIL);
+       				if (CmdFailed(ret)) continue;
 #ifdef AI_PATHFINDER_NO_90DEGREES_TURN
        				if (current->path.parent->parent != NULL) {
        					// Check if we don't make a 90degree curve
@@ -278,8 +277,8 @@ static void AyStar_AiPathFinder_GetNeighbours(AyStar *aystar, OpenListNode *curr
        				}
        				// Only destruct things if it is MP_CLEAR of MP_TREES
        				if (dir != 0) {
-       					r = DoCommandByTile(current->path.node.tile, dir, 0, DC_AUTO | DC_NO_WATER, CMD_BUILD_ROAD);
-       					if (r == CMD_ERROR) continue;
+       					ret = DoCommandByTile(current->path.node.tile, dir, 0, DC_AUTO | DC_NO_WATER, CMD_BUILD_ROAD);
+       					if (CmdFailed(ret)) continue;
        				}
        			}
 
@@ -320,8 +319,8 @@ static void AyStar_AiPathFinder_GetNeighbours(AyStar *aystar, OpenListNode *curr
     	    	if (TILES_BETWEEN(new_tile,PathFinderInfo->end_tile_tl,PathFinderInfo->end_tile_br)) break;
 
     	    	// Try building the bridge..
-    	    	r = DoCommandByTile(tile, new_tile, (0<<8) + (MAX_BRIDGES / 2), DC_AUTO, CMD_BUILD_BRIDGE);
-	    	   	if (r == CMD_ERROR) continue;
+    	    	ret = DoCommandByTile(tile, new_tile, (0<<8) + (MAX_BRIDGES / 2), DC_AUTO, CMD_BUILD_BRIDGE);
+	    	   	if (CmdFailed(ret)) continue;
     		   	// We can build a bridge here.. add him to the neighbours
    				aystar->neighbours[aystar->num_neighbours].tile = new_tile;
 	   			aystar->neighbours[aystar->num_neighbours].user_data[0] = AI_PATHFINDER_FLAG_BRIDGE + (dir << 8);
@@ -339,9 +338,9 @@ static void AyStar_AiPathFinder_GetNeighbours(AyStar *aystar, OpenListNode *curr
     		(dir == 2 && ti.tileh == 3) ||
     		(dir == 3 && ti.tileh == 9)) {
     		// Now simply check if a tunnel can be build
-    		r = DoCommandByTile(tile, (PathFinderInfo->rail_or_road?0:0x200), 0, DC_AUTO, CMD_BUILD_TUNNEL);
+    		ret = DoCommandByTile(tile, (PathFinderInfo->rail_or_road?0:0x200), 0, DC_AUTO, CMD_BUILD_TUNNEL);
     		FindLandscapeHeightByTile(&ti, _build_tunnel_endtile);
-    		if (r != CMD_ERROR && (ti.tileh == 3 || ti.tileh == 6 || ti.tileh == 9 || ti.tileh == 12)) {
+    		if (!CmdFailed(ret) && (ti.tileh == 3 || ti.tileh == 6 || ti.tileh == 9 || ti.tileh == 12)) {
     			aystar->neighbours[aystar->num_neighbours].tile = _build_tunnel_endtile;
 	   			aystar->neighbours[aystar->num_neighbours].user_data[0] = AI_PATHFINDER_FLAG_TUNNEL + (dir << 8);
 	   			aystar->neighbours[aystar->num_neighbours++].direction = 0;
diff --git a/command.h b/command.h
index 16ececd503..030bda50f0 100644
--- a/command.h
+++ b/command.h
@@ -177,6 +177,11 @@ enum {
 //#define return_cmd_error(errcode) do { _error_message=(errcode); return CMD_ERROR; } while(0)
 #define return_cmd_error(errcode) do { return CMD_ERROR | (errcode); } while (0)
 
+/**
+ * Check the return value of a DoCommand*() function
+ * @param res the resulting value from the command to be checked
+ * @return Return true if the command failed, false otherwise
+ */
 static inline bool CmdFailed(int32 res)
 {
 	// lower 16bits are the StringID of the possible error