Hide inheritance of guardt on exprt

This ensures only methods of the guardt interface are used.

Author: romainbrenguier

src/util/guard.h

@@ -16,7 +16,7 @@ Author: Daniel Kroening, [email protected]
 
 #include "std_expr.h"
 
-class guardt:public exprt
+class guardt : private exprt
 {
 public:
   guardt()
@@ -45,6 +45,16 @@ class guardt:public exprt
 
   void guard_expr(exprt &dest) const;
 
+  bool is_true() const
+  {
+    return exprt::is_true();
+  }
+
+  bool is_false() const
+  {
+    return exprt::is_false();
+  }
+
   friend guardt &operator -= (guardt &g1, const guardt &g2);
   friend guardt &operator |= (guardt &g1, const guardt &g2);
 };