<PackageReference Include="System.Reactive" Version="4.1.3" />

Aggregate<TSource, TAccumulate, TResult>

sealed class Aggregate<TSource, TAccumulate, TResult> : Producer<TResult, _<TSource, TAccumulate, TResult>>
public Aggregate(IObservable<TSource> source, TAccumulate seed, Func<TAccumulate, TSource, TAccumulate> accumulator, Func<TAccumulate, TResult> resultSelector)