-# newsdss = 'DL'.difference s.sdss newstate.sdss
-= ([MTSds sdsi $ toByteCode e\\{sdsi,sdsval=(BCValue e)}<-newsdss] ++
+# newsdss = 'DL'.difference newstate.sdss s.sdss
+| not (trace_tn $ printToString s.sdss) = undef
+| not (trace_tn $ printToString newstate.sdss) = undef
+| not (trace_tn $ printToString newsdss) = undef
+= ([MTSds sdsi e\\{sdsi,sdsval=e}<-newsdss] ++