scan

import rx.subject : SubjectObject;

auto subject = new SubjectObject!int;

auto sum = subject.scan!((a, b) => a + b)(0);
static assert(isObservable!(typeof(sum), int));

import std.array : appender;

auto buf = appender!(int[]);
auto disposable = sum.subscribe(buf);
scope (exit)
    disposable.dispose();

foreach (_; 0 .. 5)
{
    subject.put(1);
}

auto result = buf.data;
assert(result.length == 5);
import std.algorithm : equal;

assert(equal(result, [1, 2, 3, 4, 5]));