Return a KVC proxy if the receiver implements -countOfKey AND objectInKeyAtIndex:

-valueForKey:

- (id)valueForKey:(CPString)aKey
{
    var capitalizedKey = aKey.charAt(0).toUpperCase() + aKey.substring(1),
        objectAtIndexSEL = sel_getName(@"objectIn" + capitalizedKey + "AtIndex:"),
        objectsAtIndexesSEL = sel_getName(aKey + "AtIndexes:"),
        countSEL = sel_getName(@"countOf" + capitalizedKey);
    
    if ([destination respondsToSelector:countSEL] && ([destination respondsToSelector:objectAtIndexSEL] || [destination respondsToSelector:objectsAtIndexesSEL]))    
        return [destination mutableArrayValueForKey:aKey];
        
    // ...
}