NSSet *chars = [[NSSet alloc] initWithObjects: @"A", @"B", @"C", @"D", nil];
bool containA = [chars containsObject:@"A"];
//containA is 1
bool containE = [chars containsObject:@"E"];
//containE is 0
NSSet *chars2 = [[NSSet alloc] initWithObjects: @"A", @"B", nil];
bool containAll = [chars2 isSubsetOfSet:chars];
//containAll is 1