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

Author: tautschnig

src/util/guard.cpp

@@ -127,7 +127,7 @@ void guardt::add(const exprt &expr)
 
 /*******************************************************************\
 
-Function: operator -=
+Function: guardt::operator -=
 
   Inputs:
 
@@ -137,8 +137,10 @@ Function: operator -=
 
 \*******************************************************************/
 
-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

@@ -59,7 +59,7 @@ 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