diff --git a/HISTORY b/HISTORY
index e7af3c9..491fda4 100755
--- a/HISTORY
+++ b/HISTORY
@@ -1,5 +1,6 @@
 Mar. 1 2016
 Added an OS statement
+Fixed a class type detection bug with the IS statement
 Refactored platform dependent macros
 
 Feb. 29 2016
diff --git a/MY-BASIC Quick Reference.pdf b/MY-BASIC Quick Reference.pdf
index f041267..cfda98e 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 4a039ed..1718852 100755
--- a/README.md
+++ b/README.md
@@ -21,7 +21,7 @@ Why are existing script interpreters so complex? Why is it so difficult to integ
 
 ## Introduction
 
-MY-BASIC is a lightweight cross-platform easy extendable BASIC interpreter written in pure C with about 16,000 lines of source code. 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). Most advanced features are easily cuttable by modifying macros. It is aimed to be either an embeddable scripting language or a standalone interpreter. The core is pretty light; all in a C source file and an associated header file; simpleness of source file layout and tightness dependency make it feels extraordinarily tough. It's able to easily combine MY-BASIC with an existing project in C, C++, Objective-C, C#, etc. 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 dialect easily based on it.
+MY-BASIC is a lightweight cross-platform easy extendable BASIC interpreter written in pure C with less than twenty thousand lines of source code. 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). Most advanced features are easily cuttable by modifying macros. It is aimed to be either an embeddable scripting language or a standalone interpreter. The core is pretty light; all in a C source file and an associated header file; simpleness of source file layout and tightness dependency make it feels extraordinarily tough. It's able to easily combine MY-BASIC with an existing project in C, C++, Objective-C, C#, etc. 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 dialect easily based on it.
 
 ## Compatibility
 
diff --git a/core/my_basic.c b/core/my_basic.c
index 24910c6..9beadd9 100755
--- a/core/my_basic.c
+++ b/core/my_basic.c
@@ -1556,7 +1556,7 @@ static int _unlink_meta_instance(void* data, void* extra, _class_t* derived);
 static int _clone_clsss_field(void* data, void* extra, void* n);
 static bool_t _clone_class_meta_link(_class_t* meta, void* n, void* ret);
 static int _search_class_meta_function(mb_interpreter_t* s, _class_t* instance, const char* n, _routine_t** f);
-static bool_t _is_class(_class_t* instance, void* m, void* ret);
+static bool_t _is_a_class(_class_t* instance, void* m, void* ret);
 static bool_t _add_class_meta_reachable(_class_t* meta, void* ht, void* ret);
 #ifdef MB_ENABLE_COLLECTION_LIB
 static int _reflect_class_field(void* data, void* extra, void* d);
@@ -7106,14 +7106,26 @@ static int _search_class_meta_function(mb_interpreter_t* s, _class_t* instance,
 	return 0;
 }
 
-static bool_t _is_class(_class_t* instance, void* m, void* ret) {
+static bool_t _is_a_class(_class_t* instance, void* m, void* ret) {
 	/* Detect whether a class instance is inherited from another */
 	_class_t* meta = (_class_t*)m;
 	bool_t* r = (bool_t*)ret;
+	bool_t is_a = false;
 
 	mb_assert(instance && meta && ret);
 
-	*r = instance == meta;
+	do {
+		if(instance == meta) {
+			is_a = true;
+
+			break;
+		}
+		if(instance == instance->created_from)
+			break;
+		instance = instance->created_from;
+	} while(1);
+
+	*r = is_a;
 
 	return !(*r);
 }
@@ -12483,7 +12495,7 @@ static int _core_is(mb_interpreter_t* s, void** l) {
 		if(!_IS_CLASS(fst) || !_IS_CLASS(scd)) {
 			_handle_error_on_obj(s, SE_RN_CLASS_EXPECTED, s->source_file, TON(l), MB_FUNC_ERR, _exit, result);
 		}
-		_traverse_class(fst->data.instance, 0, _is_class, _META_LIST_MAX_DEPTH, true, scd->data.instance, &is_a);
+		_traverse_class(fst->data.instance, 0, _is_a_class, _META_LIST_MAX_DEPTH, true, scd->data.instance, &is_a);
 		val->type = _DT_INT;
 		val->data.integer = (int_t)is_a;
 #else /* MB_ENABLE_CLASS */
@@ -13397,8 +13409,8 @@ _retry:
 			bool_t is_a0 = false;
 			bool_t is_a1 = false;
 			if(s->last_instance && routine->instance) {
-				_traverse_class(s->last_instance->created_from, 0, _is_class, _META_LIST_MAX_DEPTH, true, routine->instance->created_from, &is_a0);
-				_traverse_class(routine->instance->created_from, 0, _is_class, _META_LIST_MAX_DEPTH, true, s->last_instance->created_from, &is_a1);
+				_traverse_class(s->last_instance->created_from, 0, _is_a_class, _META_LIST_MAX_DEPTH, true, routine->instance->created_from, &is_a0);
+				_traverse_class(routine->instance->created_from, 0, _is_a_class, _META_LIST_MAX_DEPTH, true, s->last_instance->created_from, &is_a1);
 			}
 			if(routine->instance &&
 				(!s->last_instance ||