Bug fix in the check for syntactic co-safe LTL formulas: implication/iff not allowed.

git-svn-id: https://www.prismmodelchecker.org/svn/prism/prism/trunk@10555 bbc10eb1-c90d-0410-af57-cb519fbb1720

prism/src/parser/ast/Expression.java

@@ -761,6 +761,7 @@ public abstract class Expression extends ASTElement
 	/**
 	 * Test if an expression is an LTL formula and is in positive normal form,
 	 * i.e. where negation only occurs at the level of state formulae.
+	 * This means that the operators => and <=> are also disallowed. 
 	 */
 	public static boolean isPositiveNormalFormLTL(Expression expr)
 	{
@@ -782,8 +783,16 @@ public abstract class Expression extends ASTElement
 		}
 		else if (expr instanceof ExpressionBinaryOp) {
 			ExpressionBinaryOp exprBinOp = (ExpressionBinaryOp) expr;
+			int op = exprBinOp.getOperator();
+			switch (op) {
+			// => and <=> are not allowed
+			case ExpressionBinaryOp.IMPLIES:
+			case ExpressionBinaryOp.IFF:
+				return false;
+			default:
 				return isPositiveNormalFormLTL(exprBinOp.getOperand1()) && isPositiveNormalFormLTL(exprBinOp.getOperand2());
 			}
+		}
 		else if (expr instanceof ExpressionTemporal) {
 			ExpressionTemporal exprTemp = (ExpressionTemporal) expr;
 			if (exprTemp.getOperand1() != null && !isPositiveNormalFormLTL(exprTemp.getOperand1())) {