diff --git a/stix/kernel/Collection-Array.st b/stix/kernel/Collection-Array.st
index f408101..463c684 100644
--- a/stix/kernel/Collection-Array.st
+++ b/stix/kernel/Collection-Array.st
@@ -5,6 +5,11 @@
 		^self basicSize.
 	}
 
+	#method ubound
+	{
+		^(self basicSize - 1).
+	}
+
 	#method at: anInteger
 	{
 		^self basicAt: anInteger.
@@ -17,22 +22,22 @@
 
 	#method first
 	{
-		^self at: 1.
+		^self at: 0.
 	}
 
 	#method last
 	{
-		^self at: self size.
+		^self at: (self ubound).
 	}
 
 	#method do: aBlock
 	{
-		1 to: self size do: [:i | aBlock value: (self at: i)].
+		0 to: (self ubound) do: [:i | aBlock value: (self at: i)].
 	}
 
 	#method copy: anArray
 	{
-		1 to: (anArray size) do: [:i | self at: i put: (anArray at: i) ].
+		0 to: (anArray ubound) do: [:i | self at: i put: (anArray at: i) ].
 	}
 }
 
diff --git a/stix/kernel/Process.st b/stix/kernel/Process.st
index 8498580..569073c 100644
--- a/stix/kernel/Process.st
+++ b/stix/kernel/Process.st
@@ -62,7 +62,7 @@
 	#method initialize
 	{
 		self.count := 0.
-		self.heapIndex := 0.
+		self.heapIndex := -1.
 		self.fireTime := 0.
 	}
 
@@ -126,7 +126,10 @@
 
 	#method insert: aSemaphore
 	{
-		self.size >= (self.arr size) ifTrue: [
+		| index |
+
+		index := self.size.
+		(index >= (self.arr size)) ifTrue: [
 			| newarr newsize |
 			newsize := (self.arr size) * 2.
 			newarr := Array new: newsize.
@@ -134,19 +137,19 @@
 			self.arr := newarr.
 		].
 
+		self.arr at: index put: aSemaphore.
+		aSemaphore heapIndex: index.
 		self.size := self.size + 1.
-		self.arr at: self.size put: aSemaphore.
-		aSemaphore heapIndex: self.size.
 
-		^self siftUp: self.size.
+		^self siftUp: index
 	}
 
 	#method popTop
 	{
 		| top |
 
-		top := self.arr at: 1.
-		self deleteAt: 1.
+		top := self.arr at: 0.
+		self deleteAt: 0.
 		^top
 	}
 
@@ -161,11 +164,11 @@
 		item := self.arr at: anIndex.
 		item heapIndex: -1.
 
+		self.size := self.size - 1.
 		(anIndex == self.size) 
 			ifTrue: [
 				"the last item"
 				self.arr at: self.size put: nil.
-				self.size := self.size - 1
 			]
 			ifFalse: [
 				| xitem |
@@ -174,7 +177,6 @@
 				self.arr at: anIndex put: xitem.
 				xitem heapIndex: anIndex.
 				self.arr at: self.size put: nil.
-				self.size := self.size - 1.
 
 				(xitem youngerThan: item) 
 					ifTrue: [self siftUp: anIndex ]
@@ -184,27 +186,24 @@
 
 	#method parentIndex: anIndex
 	{
-		## ^(anIndex - 1) quo: 2
-		^anIndex quo: 2
+		^(anIndex - 1) quo: 2
 	}
 
 	#method leftChildIndex: anIndex
 	{
-		## ^(anIndex * 2) + 1.
-		^(anIndex * 2).
+		^(anIndex * 2) + 1.
 	}
 
 	#method rightChildIndex: anIndex
 	{
-		## ^(anIndex * 2) + 2.
-		^(anIndex * 2) + 1.
+		 ^(anIndex * 2) + 2.
 	}
 
 	#method siftUp: anIndex
 	{
 		| pindex cindex par item stop |
 
-		(anIndex <= 1) ifTrue: [ ^anIndex ].
+		(anIndex <= 0) ifTrue: [ ^anIndex ].
 
 		pindex := anIndex.
 		item := self.arr at: anIndex.
@@ -214,7 +213,7 @@
 
 			cindex := pindex.
 
-			(cindex > 1)
+			(cindex > 0)
 				ifTrue: [
 					pindex := self parentIndex: cindex.
 					par := self.arr at: pindex.
@@ -240,8 +239,8 @@
 	{
 		| base capa cindex item |
 
-		base := (self.size quo: 2) + 1.
-		(anIndex > base) ifTrue: [^anIndex].
+		base := self.size quo: 2.
+		(anIndex >= base) ifTrue: [^anIndex].
 
 		cindex := anIndex.
 		item := self.arr at: cindex.
@@ -252,7 +251,7 @@
 			left := self leftChildIndex: cindex.
 			right := self rightChildIndex: cindex.
 
-			((right <= self.size) and: [(self.arr at: right) youngerThan: (self.arr at: left)])
+			((right < self.size) and: [(self.arr at: right) youngerThan: (self.arr at: left)])
 				ifTrue: [ younger := right ]
 				ifFalse: [ younger := left ].
 
diff --git a/stix/lib/exec.c b/stix/lib/exec.c
index a2985b2..1606fbf 100644
--- a/stix/lib/exec.c
+++ b/stix/lib/exec.c
@@ -1059,18 +1059,15 @@ static int prim_basic_at (stix_t* stix, stix_ooi_t nargs)
 	pos = ACTIVE_STACK_GET(stix, stix->sp);
 	if (stix_inttooow (stix, pos, &idx) <= 0)
 	{
-		/* integer out of range or not integer */
+		/* negative integer or not integer */
 		return 0;
 	}
-	if (idx < 1 || idx > STIX_OBJ_GET_SIZE(rcv))
+	if (idx >= STIX_OBJ_GET_SIZE(rcv))
 	{
 		/* index out of range */
 		return 0;
 	}
 
-	/* [NOTE] basicAt: and basicAt:put: uses a 1-based index. */
-	idx = idx - 1;
-
 	switch (STIX_OBJ_GET_FLAGS_TYPE(rcv))
 	{
 		case STIX_OBJ_TYPE_BYTE:
@@ -1123,10 +1120,10 @@ static int prim_basic_at_put (stix_t* stix, stix_ooi_t nargs)
 
 	if (stix_inttooow (stix, pos, &idx) <= 0)
 	{
-		/* integer out of range or not integer */
+		/* negative integer or not integer */
 		return 0;
 	}
-	if (idx < 1 || idx > STIX_OBJ_GET_SIZE(rcv))
+	if (idx >= STIX_OBJ_GET_SIZE(rcv))
 	{
 		/* index out of range */
 		return 0;
@@ -1140,9 +1137,6 @@ static int prim_basic_at_put (stix_t* stix, stix_ooi_t nargs)
 		return 0;
 	}
 
-	/* [NOTE] basicAt: and basicAt:put: uses a 1-based index. */
-	idx = idx - 1;
-
 	switch (STIX_OBJ_GET_FLAGS_TYPE(rcv))
 	{
 		case STIX_OBJ_TYPE_BYTE: