fold

import rx.subject : SubjectObject;

auto sub = new SubjectObject!int;
auto sum = sub.fold!"a+b"(0);

int result = 0;
auto disposable = sum.doSubscribe((int n) { result = n; });
scope (exit)
    disposable.dispose();

foreach (i; 1 .. 11)
    sub.put(i);

assert(result == 0);
sub.completed();
assert(result == 55);