Compose a signal corresponding to whether a source signal has sent an equal number of YESes and NOs.

gistfile1.m

[[[signal
    // Map YES to 1, NO to -1
    map:^(NSNumber* b) {
        return @(b.boolValue ? 1 : -1);
    }]
    // Running sum of yes/1's and no/-1's
    startWithScan:@0 combine:^(NSNumber* running, NSNumber* next) {
        return @(running.intValue + next.intValue);
    }]
    // A 0-sum means equal number of YES/NO's; Map 0 to YES
    map:^(NSNumber* sum) {
        return @(sum.intValue == 0);
    }];