diff --git a/HISTORY b/HISTORY
index 81bcdda..4ef2716 100755
--- a/HISTORY
+++ b/HISTORY
@@ -1,3 +1,6 @@
+May. 20 2017
+Improved stability by preventing assigning builtin boolean
+
 May. 18 2017
 Fixed some memory potential leak when popped unexpected type of argument
 
diff --git a/MY-BASIC Quick Reference.pdf b/MY-BASIC Quick Reference.pdf
index 6b26099..2f3cb7c 100644
Binary files a/MY-BASIC Quick Reference.pdf and b/MY-BASIC Quick Reference.pdf differ
diff --git a/README.md b/README.md
index be363ac..939a2ac 100755
--- a/README.md
+++ b/README.md
@@ -30,7 +30,7 @@
 
 ## Introduction
 
-MY-BASIC is a lightweight cross-platform easy extendable BASIC interpreter written in pure C with less than twenty thousand lines of source code. MY-BASIC is a dynamic typed programming language. It supports structured grammar, and implements a style of OOP called [prototype-based programming](https://en.wikipedia.org/wiki/Prototype-based_programming) paradigm, furthermore it offers a functional programming ability with [lambda abstraction](https://en.wikipedia.org/wiki/Anonymous_function). It is aimed to be either an embeddable scripting language or a standalone interpreter. The core is very lightweight; all in a C source file and an associated header file; simpleness of source file layout and tightness dependency make it extraordinarily tough. Anyone even C programming newbies could learn how to use it and add new scripting interfaces in five minutes. It's able to easily combine MY-BASIC with an existing project in C, C++, Java, Objective-C, Swift, C# and many other languages. Script driven can make your projects more powerful, elegant and neat. It's also able to learn how to build an interpreter from scratch with MY-BASIC, or build your own interpreter easily based on it.
+MY-BASIC is a lightweight cross-platform easy extendable BASIC interpreter written in pure C with about twenty thousand lines of source code. MY-BASIC is a dynamic typed programming language. It supports structured grammar, and implements a style of OOP called [prototype-based programming](https://en.wikipedia.org/wiki/Prototype-based_programming) paradigm, furthermore it offers a functional programming ability with [lambda abstraction](https://en.wikipedia.org/wiki/Anonymous_function). It is aimed to be either an embeddable scripting language or a standalone interpreter. The core is very lightweight; all in a C source file and an associated header file; simpleness of source file layout and tightness dependency make it extraordinarily tough. Anyone even C programming newbies could learn how to use it and add new scripting interfaces in five minutes. It's able to easily combine MY-BASIC with an existing project in C, C++, Java, Objective-C, Swift, C# and many other languages. Script driven can make your projects more powerful, elegant and neat. It's also possible to learn how to build an interpreter from scratch with MY-BASIC, or build your own dialect easily based on it.
 
 "MY-" in the name could be understood literally as "My" or "Make Your", it's up to you.
 
diff --git a/core/my_basic.c b/core/my_basic.c
index fd3b546..c13e0ef 100755
--- a/core/my_basic.c
+++ b/core/my_basic.c
@@ -233,7 +233,6 @@ typedef struct _ht_node_t {
 MBCONST static const char* const _ERR_DESC[] = {
 	"No error",
 	/** Common */
-	"Open MY-BASIC failed",
 	"A function with the same name already exists",
 	"A function with the name does not exists",
 	"Not supported",
@@ -245,36 +244,40 @@ MBCONST static const char* const _ERR_DESC[] = {
 	"Empty program",
 	"Program too long",
 	"Syntax error",
+	"Out of memory",
 	"Invalid data type",
 	"Type does not match",
 	"Number overflow",
 	"Invalid string",
+	"Number expected",
+	"Integer expected",
+	"String expected",
 	"Index out of bound",
 	"Cannot find with given index",
 	"Illegal bound",
 	"Too many dimensions",
-	"Operation failed",
-	"Invalid operation usage",
 	"Dimension count out of bound",
 	"Rank out of bound",
 	"Complex array required",
+	"Array identifier expected",
+	"Array subscript expected",
+	"Variable expected",
+	"Variable or array expected",
+	"Invalid identifier usage",
+	"Duplicate identifier",
 	"Label does not exist",
 	"No return point",
 	"Colon expected",
 	"Comma expected",
 	"Comma or semicolon expected",
-	"Array identifier expected",
 	"Open bracket expected",
 	"Close bracket expected",
-	"Array subscript expected",
 	"Nested too deep",
 	"Incomplete structure",
-	"Function expected",
-	"Variable or array identifier expected",
+	"Operation failed",
+	"Operator expected",
+	"Invalid operation usage",
 	"Assign operator expected",
-	"String expected",
-	"Number expected",
-	"Integer expected",
 	"ELSE statement expected",
 	"ENDIF statement expected",
 	"TO statement expected",
@@ -282,15 +285,10 @@ MBCONST static const char* const _ERR_DESC[] = {
 	"UNTIL statement expected",
 	"Loop variable expected",
 	"Jump label expected",
-	"Variable expected",
-	"Invalid identifier usage",
-	"Duplicate identifier",
-	"Operator expected",
 	"Calculation error",
+	"Invalid expression",
 	"Divide by zero",
 	"MOD by zero",
-	"Invalid expression",
-	"Out of memory",
 	"Module not match",
 	"Wrong function reached",
 	"Do not suspend in a routine",
@@ -308,13 +306,12 @@ MBCONST static const char* const _ERR_DESC[] = {
 	"HASH and COMPARE must come together",
 	"Cannot change ME",
 	"Invalid lambda",
+	"Empty collection",
 	"List expected",
 	"Collection expected",
-	"Iterator expected",
-	"Collection or iterator expected",
 	"Collection or iterator or class expected",
+	"Iterator expected",
 	"Invalid iterator",
-	"Empty collection",
 	"Referenced usertype expected",
 	"Referenced type expected",
 	"Reference count overflow",
@@ -12399,7 +12396,7 @@ int mb_init_array(struct mb_interpreter_t* s, void** l, mb_data_e t, int* d, int
 	} else if(t == MB_DT_STRING) {
 		type = _DT_STRING;
 	} else {
-		_handle_error_on_obj(s, SE_RN_NEED_COMPLEX_ARRAY, s->source_file, DON2(l), MB_FUNC_ERR, _exit, result);
+		_handle_error_on_obj(s, SE_RN_COMPLEX_ARRAY_REQUIRED, s->source_file, DON2(l), MB_FUNC_ERR, _exit, result);
 	}
 #else /* MB_SIMPLE_ARRAY */
 	type = _DT_REAL;
@@ -14341,6 +14338,9 @@ static int _core_let(mb_interpreter_t* s, void** l) {
 			evar = obj->data.variable;
 			var = _search_var_in_scope_chain(s, obj->data.variable);
 			if(var == evar) evar = 0;
+			if(var == _OBJ_BOOL_TRUE->data.variable || var == _OBJ_BOOL_FALSE->data.variable) {
+				_handle_error_on_obj(s, SE_RN_INVALID_ID_USAGE, s->source_file, DON(ast), MB_FUNC_ERR, _exit, result);
+			}
 		}
 	} else {
 		_handle_error_on_obj(s, SE_RN_VAR_OR_ARRAY_EXPECTED, s->source_file, DON(ast), MB_FUNC_ERR, _exit, result);
@@ -16960,7 +16960,7 @@ static int _std_set(mb_interpreter_t* s, void** l) {
 				_create_internal_object_from_public_value(&nv, &nobj);
 				fobj->data.variable->data = nobj;
 			} else {
-				_handle_error_on_obj(s, SE_RN_VARIABLE_EXPECTED, s->source_file, DON2(l), MB_FUNC_ERR, _exit, result);
+				_handle_error_on_obj(s, SE_RN_VAR_EXPECTED, s->source_file, DON2(l), MB_FUNC_ERR, _exit, result);
 			}
 
 			break;
@@ -17188,7 +17188,7 @@ static int _std_input(mb_interpreter_t* s, void** l) {
 		obj = (_object_t*)ast->data;
 	}
 	if(obj->type != _DT_VAR) {
-		_handle_error_on_obj(s, SE_RN_VARIABLE_EXPECTED, s->source_file, DON(ast), MB_FUNC_ERR, _exit, result);
+		_handle_error_on_obj(s, SE_RN_VAR_EXPECTED, s->source_file, DON(ast), MB_FUNC_ERR, _exit, result);
 	}
 	if(obj->data.variable->data->type == _DT_INT || obj->data.variable->data->type == _DT_REAL) {
 		_get_inputer(s)(line, sizeof(line));
diff --git a/core/my_basic.h b/core/my_basic.h
index 076d9d4..f452a7c 100755
--- a/core/my_basic.h
+++ b/core/my_basic.h
@@ -384,7 +384,6 @@ struct mb_interpreter_t;
 typedef enum mb_error_e {
 	SE_NO_ERR = 0,
 	/** Common */
-	SE_CM_OPEN_MB_FAILED,
 	SE_CM_FUNC_EXISTS,
 	SE_CM_FUNC_NOT_EXISTS,
 	SE_CM_NOT_SUPPORTED,
@@ -396,36 +395,40 @@ typedef enum mb_error_e {
 	SE_RN_EMPTY_PROGRAM,
 	SE_RN_PROGRAM_TOO_LONG,
 	SE_RN_SYNTAX_ERROR,
+	SE_RN_OUT_OF_MEMORY,
 	SE_RN_INVALID_DATA_TYPE,
 	SE_RN_TYPE_NOT_MATCH,
 	SE_RN_NUMBER_OVERFLOW,
 	SE_RN_INVALID_STRING,
+	SE_RN_NUMBER_EXPECTED,
+	SE_RN_INTEGER_EXPECTED,
+	SE_RN_STRING_EXPECTED,
 	SE_RN_INDEX_OUT_OF_BOUND,
 	SE_RN_CANNOT_FIND_WITH_GIVEN_INDEX,
 	SE_RN_ILLEGAL_BOUND,
 	SE_RN_TOO_MANY_DIMENSIONS,
-	SE_RN_OPERATION_FAILED,
-	SE_RN_INVALID_OPERATION_USAGE,
 	SE_RN_DIMENSION_COUNT_OUT_OF_BOUND,
 	SE_RN_RANK_OUT_OF_BOUND,
-	SE_RN_NEED_COMPLEX_ARRAY,
+	SE_RN_COMPLEX_ARRAY_REQUIRED,
+	SE_RN_ARRAY_IDENTIFIER_EXPECTED,
+	SE_RN_ARRAY_SUBSCRIPT_EXPECTED,
+	SE_RN_VAR_EXPECTED,
+	SE_RN_VAR_OR_ARRAY_EXPECTED,
+	SE_RN_INVALID_ID_USAGE,
+	SE_RN_DUPLICATE_ID,
 	SE_RN_LABEL_NOT_EXISTS,
 	SE_RN_NO_RETURN_POINT,
 	SE_RN_COLON_EXPECTED,
 	SE_RN_COMMA_EXPECTED,
 	SE_RN_COMMA_OR_SEMICOLON_EXPECTED,
-	SE_RN_ARRAY_IDENTIFIER_EXPECTED,
 	SE_RN_OPEN_BRACKET_EXPECTED,
 	SE_RN_CLOSE_BRACKET_EXPECTED,
-	SE_RN_ARRAY_SUBSCRIPT_EXPECTED,
 	SE_RN_NESTED_TOO_DEEP,
 	SE_RN_INCOMPLETE_STRUCTURE,
-	SE_RN_FUNCTION_EXPECTED,
-	SE_RN_VAR_OR_ARRAY_EXPECTED,
+	SE_RN_OPERATION_FAILED,
+	SE_RN_OPERATOR_EXPECTED,
+	SE_RN_INVALID_OPERATION_USAGE,
 	SE_RN_ASSIGN_OPERATOR_EXPECTED,
-	SE_RN_STRING_EXPECTED,
-	SE_RN_NUMBER_EXPECTED,
-	SE_RN_INTEGER_EXPECTED,
 	SE_RN_ELSE_EXPECTED,
 	SE_RN_ENDIF_EXPECTED,
 	SE_RN_TO_EXPECTED,
@@ -433,15 +436,10 @@ typedef enum mb_error_e {
 	SE_RN_UNTIL_EXPECTED,
 	SE_RN_LOOP_VAR_EXPECTED,
 	SE_RN_JUMP_LABEL_EXPECTED,
-	SE_RN_VARIABLE_EXPECTED,
-	SE_RN_INVALID_ID_USAGE,
-	SE_RN_DUPLICATE_ID,
-	SE_RN_OPERATOR_EXPECTED,
 	SE_RN_CALCULATION_ERROR,
+	SE_RN_INVALID_EXPRESSION,
 	SE_RN_DIVIDE_BY_ZERO,
 	SE_RN_MOD_BY_ZERO,
-	SE_RN_INVALID_EXPRESSION,
-	SE_RN_OUT_OF_MEMORY,
 	SE_RN_MODULE_NOT_MATCH,
 	SE_RN_WRONG_FUNCTION_REACHED,
 	SE_RN_DO_NOT_SUSPEND_IN_A_ROUTINE,
@@ -459,13 +457,12 @@ typedef enum mb_error_e {
 	SE_RN_HASH_AND_COMPARE_MUST_COME_TOGETHER,
 	SE_RN_CANNOT_CHANGE_ME,
 	SE_RN_INVALID_LAMBDA,
+	SE_RN_EMPTY_COLLECTION,
 	SE_RN_LIST_EXPECTED,
 	SE_RN_COLLECTION_EXPECTED,
-	SE_RN_ITERATOR_EXPECTED,
-	SE_RN_COLLECTION_OR_ITERATOR_EXPECTED,
 	SE_RN_COLLECTION_OR_ITERATOR_OR_CLASS_EXPECTED,
+	SE_RN_ITERATOR_EXPECTED,
 	SE_RN_INVALID_ITERATOR,
-	SE_RN_EMPTY_COLLECTION,
 	SE_RN_REFERENCED_USERTYPE_EXPECTED,
 	SE_RN_REFERENCED_TYPE_EXPECTED,
 	SE_RN_REFERENCE_COUNT_OVERFLOW,