merge

import rx.algorithm.groupby : groupBy;
import rx.algorithm.map : map;
import rx.algorithm.fold : fold;
import rx.subject : SubjectObject, CounterObserver;

auto subject = new SubjectObject!int;
auto counted = subject.groupBy!(n => n % 10).map!(o => o.fold!((a, b) => a + 1)(0)).merge();

auto counter = new CounterObserver!int;

auto disposable = counted.subscribe(counter);

subject.put(0);
subject.put(0);
assert(counter.putCount == 0);
subject.completed();
assert(counter.putCount == 1);
assert(counter.lastValue == 2);