Statistical MC: Check bounds for CTMC bounded until

Throw an error if the bounds are negative or upper bound smaller than
lower bound (similar to the checks in the other engines / samplers).

prism/src/simulator/sampler/SamplerBoundedUntilCont.java

@@ -55,8 +55,25 @@ public class SamplerBoundedUntilCont extends SamplerBoolean
 			throw new PrismException("Error creating Sampler");
 		left = expr.getOperand1();
 		right = expr.getOperand2();
+
 		lb = expr.getLowerBound() == null ? 0.0 : expr.getLowerBound().evaluateDouble();
-		ub = expr.getUpperBound() == null ? Double.POSITIVE_INFINITY : expr.getUpperBound().evaluateDouble();
+		if (lb < 0) {
+			throw new PrismException("Invalid lower bound " + lb + " in time-bounded until formula");
+		}
+
+		if (expr.getUpperBound() == null) {
+			ub = Double.POSITIVE_INFINITY;
+		} else {
+			ub = expr.getUpperBound().evaluateDouble();
+			if (ub < 0 || (ub == 0 && expr.upperBoundIsStrict())) {
+				String bound = (expr.upperBoundIsStrict() ? "<" : "<=") + ub;
+				throw new PrismException("Invalid upper bound " + bound + " in time-bounded until formula");
+			}
+			if (ub < lb) {
+				throw new PrismException("Upper bound must exceed lower bound in time-bounded until formula");
+			}
+		}
+
 		// Initialise sampler info
 		reset();
 		resetStats();