Add check that POMDP observables are valid variable references.

Also fix a few test cases (with erroneous extra  observables).

prism/src/explicit/ConstructModel.java

@@ -236,6 +236,11 @@ public class ConstructModel extends PrismComponent
 		if (!justReach && modelType.partiallyObservable()) {
 			List<String> allVars = modelGen.getVarNames();
 			observableVars = modelGen.getObservableVars();
+			for (String obsVar : observableVars) {
+				if (!allVars.contains(obsVar)) {
+					throw new PrismException("Observable " + obsVar + " is not a variable");
+				}
+			}
 			unobservableVars = new ArrayList<>();
 			for (String varName : allVars) {
 				if (!observableVars.contains(varName)) {

prism/tests/poptas/pump_popta.prism

@@ -13,7 +13,7 @@ popta
 
 // can see the state of the pump and its local variables and all clocks
 observables
-	p, messages, l, m, guess, correct, x, y
+	p, l, m, guess, correct, x, y
 endobservables
 
 // delays for high sending 0 and 1 (both need to be >1)

prism/tests/poptas/pump_popta_deadline.prism

@@ -2,7 +2,7 @@ pomdp
 
 // can see the state of the pump and its local variables and all clocks
 observables
-	p, messages, l, m, guess, correct, x, y, t
+	p, l, m, guess, correct, x, y, t
 endobservables
 
 label "invariants" = (p=0=>true)&(p=1=>y<=1)&(p=2=>y<=0)&(l=0=>x<=0)&(l=1=>x<=Tout)&(l=2=>x<=0)&(l=3=>true);