Override @property setter and infinite loop

There is Class A with:

@interface ClassA : NSObject {
}
@property (nonatomic, assign) id prop1;
@end

@implementation
@synthesize prop1;
@end

then I have subclass

@interface ClassB : ClassA {
}
@end

@implementation

- (id)init {
    self = [super init];
    if (self) {
    }
    return self;
}

//This is infinite loop
- (void) setProp1:(id)aProp
{
    self.prop1 = aProp;
}
@end

and this is infinite loop because setProp1 from ClassB calls [ClassB setProp1:val] from within ClassB.

I've already tried call [super setProp1] but this

How to overwrite @property and assign value inside overwritten setter ? And let's assume I can't modify ClassA.