diff --git a/moo/kernel/Apex.moo b/moo/kernel/Apex.moo
index 10146f8..fc0a2d3 100644
--- a/moo/kernel/Apex.moo
+++ b/moo/kernel/Apex.moo
@@ -122,6 +122,7 @@ extend Apex
 
 	method(#dual,#primitive) basicAt: index.
 	method(#dual,#primitive) basicAt: index put: value.
+	method(#dual,#primitive) basicAt: index test: test oldvalue put: newvalue.
 
 	method(#dual,#primitive) basicFillFrom: sindex with: value count: count.
 	method(#dual,#primitive) basicShiftFrom: sindex to: dindex count: count.
diff --git a/moo/kernel/Except.moo b/moo/kernel/Except.moo
index 4f5d2db..69c72e0 100644
--- a/moo/kernel/Except.moo
+++ b/moo/kernel/Except.moo
@@ -174,7 +174,8 @@ extend Context
 			if (eb notNil)
 			{
 				/* position of the temporary variable in the ensureBlock that indicates
-				 * if the block has been evaluated */
+				 * if the block has been evaluated. see the method BlockContext>>ensure:.
+				 * it is the position of the last temporary variable of the method */
 				pending_pos := ctx basicSize - 1. 
 				if (ctx basicAt: pending_pos)
 				{
diff --git a/moo/lib/exec.c b/moo/lib/exec.c
index 8f70339..2a222cd 100644
--- a/moo/lib/exec.c
+++ b/moo/lib/exec.c
@@ -4048,6 +4048,7 @@ static pf_t pftab[] =
 	{ "Apex_addToBeFinalized",                 { pf_add_to_be_finalized,                  0, 0 } },
 	{ "Apex_basicAt:",                         { moo_pf_basic_at,                         1, 1 } },
 	{ "Apex_basicAt:put:",                     { moo_pf_basic_at_put,                     2, 2 } },
+	{ "Apex_basicAt:test:put:",                { moo_pf_basic_at_test_put,                3, 3 } },
 	{ "Apex_basicFillFrom:with:count:",        { moo_pf_basic_fill,                       3, 3 } },
 	{ "Apex_basicNew",                         { moo_pf_basic_new,                        0, 0 } },
 	{ "Apex_basicNew:",                        { moo_pf_basic_new,                        1, 1 } },
diff --git a/moo/lib/moo-prv.h b/moo/lib/moo-prv.h
index 0869807..78cef41 100644
--- a/moo/lib/moo-prv.h
+++ b/moo/lib/moo-prv.h
@@ -1614,6 +1614,7 @@ moo_pfrc_t moo_pf_class (moo_t* moo, moo_mod_t* mod, moo_ooi_t nargs);
 moo_pfrc_t moo_pf_basic_size (moo_t* moo, moo_mod_t* mod, moo_ooi_t nargs);
 moo_pfrc_t moo_pf_basic_at (moo_t* moo, moo_mod_t* mod, moo_ooi_t nargs);
 moo_pfrc_t moo_pf_basic_at_put (moo_t* moo, moo_mod_t* mod, moo_ooi_t nargs);
+moo_pfrc_t moo_pf_basic_at_test_put (moo_t* moo, moo_mod_t* mod, moo_ooi_t nargs);
 moo_pfrc_t moo_pf_basic_fill (moo_t* moo, moo_mod_t* mod, moo_ooi_t nargs);
 moo_pfrc_t moo_pf_basic_shift (moo_t* moo, moo_mod_t* mod, moo_ooi_t nargs);
 
diff --git a/moo/lib/pf-basic.c b/moo/lib/pf-basic.c
index 0b17f48..f937a24 100644
--- a/moo/lib/pf-basic.c
+++ b/moo/lib/pf-basic.c
@@ -378,7 +378,7 @@ moo_pfrc_t moo_pf_basic_at_put (moo_t* moo, moo_mod_t* mod, moo_ooi_t nargs)
 	pos = MOO_STACK_GETARG(moo, nargs, 0);
 	val = MOO_STACK_GETARG(moo, nargs, 1);
 
-	if (moo_inttooow (moo, pos, &idx) <= 0)
+	if (moo_inttooow(moo, pos, &idx) <= 0)
 	{
 		/* negative integer or not integer */
 		moo_seterrbfmt (moo, MOO_EINVAL, "invalid position - %O", pos);
@@ -428,7 +428,106 @@ moo_pfrc_t moo_pf_basic_at_put (moo_t* moo, moo_mod_t* mod, moo_ooi_t nargs)
 		{
 			moo_oow_t w;
 
-			if (moo_inttooow (moo, val, &w) <= 0)
+			if (moo_inttooow(moo, val, &w) <= 0)
+			{
+				/* the value is not a number, out of range, or negative */
+				moo_seterrbfmt (moo, MOO_EINVAL, "value not a word integer - %O", val);
+				return MOO_PF_FAILURE;
+			}
+			MOO_OBJ_SET_WORD_VAL (rcv, idx, MOO_OOP_TO_SMOOI(val));
+			break;
+		}
+
+		case MOO_OBJ_TYPE_OOP:
+			MOO_STORE_OOP (moo, MOO_OBJ_GET_OOP_PTR(rcv, idx), val);
+			break;
+
+		default:
+			moo_seterrnum (moo, MOO_EINTERN);
+			return MOO_PF_HARD_FAILURE;
+	}
+
+/* TODO: return receiver or value? */
+	MOO_STACK_SETRET (moo, nargs, val);
+	return MOO_PF_SUCCESS;
+}
+
+moo_pfrc_t moo_pf_basic_at_test_put (moo_t* moo, moo_mod_t* mod, moo_ooi_t nargs)
+{
+	moo_oop_t rcv, pos, oldval, newval;
+	moo_oow_t idx;
+
+	MOO_ASSERT (moo, nargs == 3);
+
+	rcv = MOO_STACK_GETRCV(moo, nargs);
+	if (!MOO_OOP_IS_POINTER(rcv))
+	{
+		/* the receiver is a special numeric object, not a normal pointer */
+		moo_seterrbfmt (moo, MOO_EMSGRCV, "receiver not indexable - %O", rcv);
+		return MOO_PF_FAILURE;
+	}
+
+	if (MOO_OBJ_GET_FLAGS_RDONLY(rcv))
+	{
+		moo_seterrbfmt (moo, MOO_EPERM, "not allowed to change a read-only object - %O", rcv);
+		return MOO_PF_FAILURE;
+	}
+
+	pos = MOO_STACK_GETARG(moo, nargs, 0);
+	oldval = MOO_STACK_GETARG(moo, nargs, 1);
+	newval = MOO_STACK_GETARG(moo, nargs, 1);
+
+	if (moo_inttooow(moo, pos, &idx) <= 0)
+	{
+		/* negative integer or not integer */
+		moo_seterrbfmt (moo, MOO_EINVAL, "invalid position - %O", pos);
+		return MOO_PF_FAILURE;
+	}
+	if (idx >= MOO_OBJ_GET_SIZE(rcv))
+	{
+		/* index out of range */
+		moo_seterrbfmt (moo, MOO_ERANGE, "position out of bound - %zu", idx);
+		return MOO_PF_FAILURE;
+	}
+
+	switch (MOO_OBJ_GET_FLAGS_TYPE(rcv))
+	{
+		case MOO_OBJ_TYPE_BYTE:
+			if (!MOO_OOP_IS_SMOOI(val))
+			{
+				moo_seterrbfmt (moo, MOO_EINVAL, "value not a byte - %O", val);
+				return MOO_PF_FAILURE;
+			}
+/* TOOD: must I check the range of the value? */
+			MOO_OBJ_SET_BYTE_VAL (rcv, idx, MOO_OOP_TO_SMOOI(val));
+			break;
+
+		case MOO_OBJ_TYPE_CHAR:
+			if (!MOO_OOP_IS_CHAR(val))
+			{
+				moo_seterrbfmt (moo, MOO_EINVAL, "value not a character - %O", val);
+				return MOO_PF_FAILURE;
+			}
+			MOO_OBJ_SET_CHAR_VAL (rcv, idx, MOO_OOP_TO_CHAR(val));
+			break;
+
+		case MOO_OBJ_TYPE_HALFWORD:
+			if (!MOO_OOP_IS_SMOOI(val))
+			{
+				/* the value is not a number */
+				moo_seterrbfmt (moo, MOO_EINVAL, "value not a half-word integer - %O", val);
+				return MOO_PF_FAILURE;
+			}
+
+			/* if the small integer is too large, it will get truncated */
+			MOO_OBJ_SET_HALFWORD_VAL (rcv, idx, MOO_OOP_TO_SMOOI(val));
+			break;
+
+		case MOO_OBJ_TYPE_WORD:
+		{
+			moo_oow_t w;
+
+			if (moo_inttooow(moo, val, &w) <= 0)
 			{
 				/* the value is not a number, out of range, or negative */
 				moo_seterrbfmt (moo, MOO_EINVAL, "value not a word integer - %O", val);