fix DigitalClock manipulation

prism/src/pta/DigitalClocks.java

@@ -383,7 +383,8 @@ public class DigitalClocks
 				public Object visit(ExpressionTemporal e) throws PrismLangException
 				{
 					// Push (new) time bound into target
-					e.getBounds().getTimeBoundForContinuousTime().setUpperBound(null);
+					// TODO (joachim): make more robust
+					e.getBounds().bounds.remove(e.getBounds().getTimeBoundForContinuousTime());
 					Expression targetNew = Expression.And(e.getOperand2().deepCopy(), boundNew);
 					e.setOperand2(targetNew);
 					return e;