Index: libcfa/src/bits/containers.hfa
===================================================================
--- libcfa/src/bits/containers.hfa	(revision 2856982c6db55f1fdfb3012b77b490d999668ae8)
+++ libcfa/src/bits/containers.hfa	(revision 0b1ca470cb4e37c87b603e45955c6cb1b5962233)
@@ -152,4 +152,5 @@
 
 		void append( __queue(T) & this, T * val ) with(this) {
+			verify(get_next( *val ) == 0p);
 			verify(this.tail != 0p);
 			verify(*this.tail == 1p);