guardt::operator-=: use member function instead of friend

Author: tautschnig

src/util/guard.cpp

@@ -94,8 +94,10 @@ void guardt::add(const exprt &expr)
     op.push_back(expr);
 }
 
-guardt &operator -= (guardt &g1, const guardt &g2)
+guardt& guardt::operator -= (const guardt &g2)
 {
+  guardt &g1=*this;
+
   if(g1.id()!=ID_and || g2.id()!=ID_and)
     return g1;
 

src/util/guard.h

@@ -62,7 +62,7 @@ class guardt:public exprt
   void make_false();
 #endif
 
-  friend guardt &operator -= (guardt &g1, const guardt &g2);
+  guardt& operator -= (const guardt &g2);
   guardt& logical_or(const guardt &g2, const namespacet &ns);
 
 #if 0