Index: libcfa/src/bits/collection.hfa
===================================================================
--- libcfa/src/bits/collection.hfa	(revision 53449a48dee3324f96932ed8c4befe2756837314)
+++ libcfa/src/bits/collection.hfa	(revision 19de7864a47b37585af2cc6f6108a0337c2a37c1)
@@ -35,11 +35,12 @@
 	// 		return (T *)Next( (Colable *)n );
 	// 	}
-
-	// 	bool listed( T * n ) {
-	// 		return Next( (Colable *)n ) != 0p;
-	// 	}
 	// } // distribution
 } // distribution
 
+forall( dtype T | { T *& Next ( T * ); } ) {
+	bool listed( T * n ) {
+		return Next( n ) != 0p;
+	}
+}
 
 struct Collection {
Index: libcfa/src/bits/queue.hfa
===================================================================
--- libcfa/src/bits/queue.hfa	(revision 53449a48dee3324f96932ed8c4befe2756837314)
+++ libcfa/src/bits/queue.hfa	(revision 19de7864a47b37585af2cc6f6108a0337c2a37c1)
@@ -3,5 +3,5 @@
 #include "bits/collection.hfa"
 
-forall( dtype T | { T *& Next ( T * ); bool listed ( T * ); } ) {
+forall( dtype T | { T *& Next ( T * ); } ) {
 	struct Queue {
 		inline Collection;								// Plan 9 inheritance
@@ -142,5 +142,5 @@
 } // distribution
 
-forall( dtype T | { T *& Next ( T * ); bool listed ( T * ); } ) {
+forall( dtype T | { T *& Next ( T * ); } ) {
 	struct QueueIter {
 		inline ColIter;									// Plan 9 inheritance
Index: libcfa/src/bits/sequence.hfa
===================================================================
--- libcfa/src/bits/sequence.hfa	(revision 53449a48dee3324f96932ed8c4befe2756837314)
+++ libcfa/src/bits/sequence.hfa	(revision 19de7864a47b37585af2cc6f6108a0337c2a37c1)
@@ -36,5 +36,5 @@
 } // distribution
 
-forall( dtype T | { T *& Back ( T * ); T *& Next ( T * ); bool listed ( T * ); } ) {
+forall( dtype T | { T *& Back ( T * ); T *& Next ( T * ); } ) {
 	struct Sequence {
 		inline Collection;								// Plan 9 inheritance
@@ -216,5 +216,5 @@
 } // distribution
 
-forall( dtype T | { T *& Back ( T * ); T *& Next ( T * ); bool listed ( T * ); } ) {
+forall( dtype T | { T *& Back ( T * ); T *& Next ( T * ); } ) {
 	// SeqIter(T) is used to iterate over a Sequence(T) in head-to-tail order.
 	struct SeqIter {
Index: libcfa/src/bits/stack.hfa
===================================================================
--- libcfa/src/bits/stack.hfa	(revision 53449a48dee3324f96932ed8c4befe2756837314)
+++ libcfa/src/bits/stack.hfa	(revision 19de7864a47b37585af2cc6f6108a0337c2a37c1)
@@ -3,5 +3,5 @@
 #include "bits/collection.hfa"
 
-forall( dtype T | { T *& Next ( T * ); bool listed ( T * ); } ) {
+forall( dtype T | { T *& Next ( T * ); } ) {
 	struct Stack {
 		inline Collection;								// Plan 9 inheritance
@@ -58,5 +58,5 @@
 
 
-forall( dtype T | { T *& Next ( T * ); bool listed ( T * ); } ) {
+forall( dtype T | { T *& Next ( T * ); } ) {
 	struct StackIter {
 		inline ColIter;									// Plan 9 inheritance