diff --git a/lib/exec.c b/lib/exec.c
index 39cdc14..93fff2a 100644
--- a/lib/exec.c
+++ b/lib/exec.c
@@ -3934,12 +3934,16 @@ static int execute (hcl_t* hcl)
 						if (HCL_OBJ_GET_FLAGS_KERNEL(class_obj) == 1)
 						{
 							/* check if the new definition is compatible with kernel definition */
-							hcl_ooi_t spec, selfspec;
+							hcl_ooi_t spec, selfspec, nivars_super, nivars_super_real;
 
 							spec = HCL_OOP_TO_SMOOI(class_obj->spec);
 							selfspec = HCL_OOP_TO_SMOOI(class_obj->selfspec);
+							nivars_super = HCL_OOP_TO_SMOOI(class_obj->nivars_super);
+							nivars_super_real = HCL_IS_NIL(hcl, superclass)? 0: HCL_OOP_TO_SMOOI(((hcl_oop_class_t)superclass)->nivars_super);
+#if 0
 hcl_logbfmt (hcl, HCL_LOG_STDERR, ">>>%O c->sc=%O sc=%O b2=%d b3=%d nivars=%d ncvars=%d<<<\n", class_obj, class_obj->superclass, superclass, b2, b3, (int)HCL_CLASS_SPEC_NAMED_INSTVARS(spec), (int)HCL_CLASS_SELFSPEC_CLASSVARS(spec));
-							if (class_obj->superclass != superclass || HCL_CLASS_SPEC_NAMED_INSTVARS(spec) != b2 || HCL_CLASS_SELFSPEC_CLASSVARS(selfspec) != b3)
+#endif
+							if (class_obj->superclass != superclass || HCL_CLASS_SPEC_NAMED_INSTVARS(spec) != b2 || HCL_CLASS_SELFSPEC_CLASSVARS(selfspec) != b3 || nivars_super != nivars_super_real)
 							{
 								hcl_seterrbfmt (hcl, HCL_EPERM, "incompatible redefintion of %.*js", HCL_OBJ_GET_SIZE(class_name), HCL_OBJ_GET_CHAR_SLOT(class_name));
 								if (do_throw_with_internal_errmsg(hcl, fetched_instruction_pointer) >= 0) break;
diff --git a/lib/gc.c b/lib/gc.c
index de29f5c..6518eae 100644
--- a/lib/gc.c
+++ b/lib/gc.c
@@ -1468,6 +1468,7 @@ static int ignite_1 (hcl_t* hcl)
 		HCL_OBJ_SET_CLASS (hcl->c_class, (hcl_oop_t)hcl->c_class);
 	}
 
+	/* create class objects except Class */
 	for (i = 0; i < HCL_COUNTOF(kernel_classes); i++)
 	{
 		hcl_oop_class_t tmp;
@@ -1496,6 +1497,20 @@ static int ignite_1 (hcl_t* hcl)
 		*(hcl_oop_class_t*)((hcl_uint8_t*)hcl + kernel_classes[i].offset) = tmp;
 	}
 
+	/* update the superclass field */
+	for (i = 0; i < HCL_COUNTOF(kernel_classes); i++)
+	{
+		int skci;
+		skci = kernel_classes[i].superclass_kci;
+		if (skci >= 0)
+		{
+			hcl_oop_class_t* x, * y;
+			x = (hcl_oop_class_t*)((hcl_uint8_t*)hcl + kernel_classes[i].offset);
+			y = (hcl_oop_class_t*)((hcl_uint8_t*)hcl + kernel_classes[skci].offset);
+			(*x)->superclass = (hcl_oop_t)*y;
+		}
+	}
+
 #if 0
 	/* an instance of a method class stores byte codes in the trailer space.
 	 * unlike other classes with trailer size set, the size of the trailer
diff --git a/mod/core.c b/mod/core.c
index 6048f3b..d7d31fd 100644
--- a/mod/core.c
+++ b/mod/core.c
@@ -48,16 +48,217 @@ static hcl_pfrc_t pf_core_get_class_name (hcl_t* hcl, hcl_mod_t* mod, hcl_ooi_t
 	return HCL_PF_SUCCESS;
 }
 
+static hcl_pfrc_t pf_core_size (hcl_t* hcl, hcl_mod_t* mod, hcl_ooi_t nargs)
+{
+	hcl_oop_oop_t src;
+	hcl_oop_t size;
+
+	src = (hcl_oop_oop_t)HCL_STACK_GETARG(hcl, nargs, 0);
+
+	if (!HCL_OOP_IS_POINTER(src))
+	{
+		hcl_seterrbfmt (hcl, HCL_EINVAL, "source not sizable - %O", src);
+		return HCL_PF_FAILURE;
+	}
+
+	size = hcl_oowtoint(hcl, HCL_OBJ_GET_SIZE(src));
+	if (!size) return HCL_PF_FAILURE;
+
+	HCL_STACK_SETRET (hcl, nargs, size);
+	return HCL_PF_SUCCESS;
+}
+
+static hcl_pfrc_t pf_core_slice (hcl_t* hcl, hcl_mod_t* mod, hcl_ooi_t nargs)
+{
+	hcl_oop_t src, slice, a1, a2;
+	hcl_ooi_t size;
+	hcl_ooi_t pos;
+	hcl_ooi_t len;
+	hcl_ooi_t i;
+
+	src = HCL_STACK_GETARG(hcl, nargs, 0);
+	a1 = HCL_STACK_GETARG(hcl, nargs, 1);
+	a2 = HCL_STACK_GETARG(hcl, nargs, 2);
+
+	if (!HCL_OOP_IS_POINTER(src))
+	{
+	unsliceable:
+		hcl_seterrbfmt (hcl, HCL_EINVAL, "source not sliceable - %O", src);
+		return HCL_PF_FAILURE;
+	}
+
+	if (!HCL_OOP_IS_SMOOI(a1))
+	{
+		hcl_seterrbfmt (hcl, HCL_EINVAL, "position not numeric - %O", a1);
+		return HCL_PF_FAILURE;
+	}
+	if (!HCL_OOP_IS_SMOOI(a2))
+	{
+		hcl_seterrbfmt (hcl, HCL_EINVAL, "length not numeric - %O", a2);
+		return HCL_PF_FAILURE;
+	}
+
+	size = HCL_OBJ_GET_SIZE(src);
+	pos = HCL_OOP_TO_SMOOI(a1);
+	len = HCL_OOP_TO_SMOOI(a2);
+
+	if (pos < 0) pos = 0;
+	else if (pos >= size) pos = size;
+	if (len >= size - pos) len = size - pos;
+
+/* TODO: check if the object is an indexable object from the class spec... */
+	/* use HCL_OBJ_GET_CLASS() instead of HCL_CLASSOF() as we know it's an object */
+	slice = hcl_instantiate(hcl, (hcl_oop_class_t)HCL_OBJ_GET_CLASS(src), HCL_NULL, len);
+	if (HCL_UNLIKELY(!slice)) return HCL_PF_FAILURE;
+
+/* OR if add by the number of fixed fields??? */
+	switch (HCL_OBJ_GET_FLAGS_TYPE(src))
+	{
+		case HCL_OBJ_TYPE_OOP:
+			for (i = 0; i < len; i++) HCL_OBJ_GET_OOP_VAL(slice, i) = HCL_OBJ_GET_OOP_VAL(src, pos + i);
+			break;
+
+		case HCL_OBJ_TYPE_CHAR:
+			for (i = 0; i < len; i++) HCL_OBJ_GET_CHAR_VAL(slice, i) = HCL_OBJ_GET_CHAR_VAL(src, pos + i);
+			break;
+
+		case HCL_OBJ_TYPE_BYTE:
+			for (i = 0; i < len; i++) HCL_OBJ_GET_BYTE_VAL(slice, i) = HCL_OBJ_GET_BYTE_VAL(src, pos + i);
+			break;
+
+		case HCL_OBJ_TYPE_HALFWORD:
+			for (i = 0; i < len; i++) HCL_OBJ_GET_HALFWORD_VAL(slice, i) = HCL_OBJ_GET_HALFWORD_VAL(src, pos + i);
+			break;
+
+		case HCL_OBJ_TYPE_WORD:
+			for (i = 0; i < len; i++) HCL_OBJ_GET_WORD_VAL(slice, i) = HCL_OBJ_GET_WORD_VAL(src, pos + i);
+			break;
+
+		default:
+			goto unsliceable;
+			break;
+	}
+
+	HCL_STACK_SETRET (hcl, nargs, slice);
+	return HCL_PF_SUCCESS;
+}
+
+static hcl_pfrc_t pf_core_get (hcl_t* hcl, hcl_mod_t* mod, hcl_ooi_t nargs)
+{
+	hcl_oop_t obj, val;
+	hcl_oop_t pos;
+	hcl_oow_t index;
+
+	obj = HCL_STACK_GETARG(hcl, nargs, 0);
+	pos = HCL_STACK_GETARG(hcl, nargs, 1);
+
+	if (!HCL_OOP_IS_POINTER(obj))
+	{
+	unindexable:
+		hcl_seterrbfmt (hcl, HCL_EINVAL, "object not indexable - %O", obj);
+		return HCL_PF_FAILURE;
+	}
+
+	if (!HCL_OOP_IS_SMOOI(pos))
+	{
+		hcl_seterrbfmt (hcl, HCL_EINVAL, "position not numeric - %O", pos);
+		return HCL_PF_FAILURE;
+	}
+	index = HCL_OOP_TO_SMOOI(pos);
+	if (index < 0 || index >= HCL_OBJ_GET_SIZE(obj))
+	{
+		hcl_seterrbfmt (hcl, HCL_EINVAL, "position(%O) out of range - 0 and %zu", pos, (hcl_oow_t)HCL_OBJ_GET_SIZE(obj) - 1);
+		return HCL_PF_FAILURE;
+	}
+
+	switch (HCL_OBJ_GET_FLAGS_TYPE(obj))
+	{
+		case HCL_OBJ_TYPE_OOP:
+			val = HCL_OBJ_GET_OOP_VAL(obj, index);
+			break;
+
+		case HCL_OBJ_TYPE_CHAR:
+		{
+			hcl_ooch_t c;
+			c = HCL_OBJ_GET_CHAR_VAL(obj, index);
+			val = HCL_CHAR_TO_OOP(c);
+			break;
+		}
+
+		case HCL_OBJ_TYPE_BYTE:
+		{
+			hcl_ooi_t b;
+			b = HCL_OBJ_GET_BYTE_VAL(obj, index);
+			val = HCL_SMOOI_TO_OOP(b);
+			break;
+		}
+
+		case HCL_OBJ_TYPE_HALFWORD:
+			val = hcl_oowtoint(hcl, HCL_OBJ_GET_HALFWORD_VAL(obj, index));
+			if (HCL_UNLIKELY(!val)) return HCL_PF_FAILURE;
+			break;
+
+		case HCL_OBJ_TYPE_WORD:
+			val = hcl_oowtoint(hcl, HCL_OBJ_GET_WORD_VAL(obj, index));
+			if (HCL_UNLIKELY(!val)) return HCL_PF_FAILURE;
+			break;
+
+		default:
+			goto unindexable;
+			break;
+	}
+
+	HCL_STACK_SETRET (hcl, nargs, val);
+	return HCL_PF_SUCCESS;
+}
+
+#if 0
+static hcl_pfrc_t pf_core_put (hcl_t* hcl, hcl_mod_t* mod, hcl_ooi_t nargs)
+{
+	hcl_oop_t obj;
+	hcl_oop_t pos, val;
+	hcl_oow_t index;
+
+	obj = HCL_STACK_GETARG(hcl, nargs, 0);
+	pos = HCL_STACK_GETARG(hcl, nargs, 1);
+	val = HCL_STACK_GETARG(hcl, nargs, 2);
+
+	if (!HCL_OOP_IS_POINTER(obj))
+	{
+	unindexable:
+		hcl_seterrbfmt (hcl, HCL_EINVAL, "object not indexable - %O", obj);
+		return HCL_PF_FAILURE;
+	}
+
+	if (!HCL_IS_ARRAY(hcl,obj))
+	{
+		hcl_seterrbfmt (hcl, HCL_EINVAL, "parameter not an array - %O", obj);
+		return HCL_PF_FAILURE;
+	}
+	if (!HCL_OOP_IS_SMOOI(pos))
+	{
+		hcl_seterrbfmt (hcl, HCL_EINVAL, "position not numeric - %O", pos);
+		return HCL_PF_FAILURE;
+	}
+	index = HCL_OOP_TO_SMOOI(pos);
+
+	obj->slot[index] = val;
+	HCL_STACK_SETRET (hcl, nargs, val);
+	return HCL_PF_SUCCESS;
+}
+#endif
+
 static hcl_pfinfo_t pfinfos[] =
 {
 	{ { 'c','l','a','s','s','_','n','a','m','e','\0' },  { HCL_PFBASE_FUNC, pf_core_get_class_name, 1,  1 } },
-/*
+	{ { 'g','e','t','\0' },              { HCL_PFBASE_FUNC, pf_core_get,     2,  2 } },
 	{ { 'l','e','n','g','t','h','\0' },  { HCL_PFBASE_FUNC, pf_core_size,    1,  1 } },
+/*
 	{ { 'n','e','w','\0' },              { HCL_PFBASE_FUNC, pf_core_new,     1,  1 } },
 	{ { 'p','u','t','\0' },              { HCL_PFBASE_FUNC, pf_core_put,     3,  3 } },
+*/
 	{ { 's','i','z','e','\0' },          { HCL_PFBASE_FUNC, pf_core_size,    1,  1 } },
 	{ { 's','l','i','c','e','\0' },      { HCL_PFBASE_FUNC, pf_core_slice,   3,  3 } }
-*/
 };
 
 /* ------------------------------------------------------------------------ */
diff --git a/src/kernel.hcl b/src/kernel.hcl
index d2c9fa6..cc9dcb8 100644
--- a/src/kernel.hcl
+++ b/src/kernel.hcl
@@ -10,21 +10,25 @@ class Collection :: Object {
 class IndexedCollection :: Collection {
 }
 
-class FixedSizedCollection :: Collection {
+class FixedSizedCollection :: IndexedCollection {
 }
 
 class Array :: FixedSizedCollection {
 }
 
-class String :: Array {
+class String :: FixedSizedCollection {
 }
 
 fun Collection:length() {
-	return (arr.length self)
+	return (core.length self)
 }
 
 fun Collection:slice(index count) {
-	return (arr.slice self index count)
+	return (core.slice self index count)
+}
+
+fun Collection:at(index) {
+	return (core.get self index)
 }
 
 fun Class:name() {
@@ -50,3 +54,5 @@ fun Class:name() {
 
 
 printf "string length %d\n" ("aaaa":length)
+printf "substring [%s]\n" ("abcdefghijklmn":slice 5 6)
+printf "substring [%c]\n" ("abcdefghijklmn":at 14)