While MergeScanOperator has implementation to accept concurrency parameter
constructor(private project: (acc: R, x: T) => Observable<R>,
private seed: R,
private concurrent: number = Number.POSITIVE_INFINITY)
type definition of mergeScan does not allow to use it.
mergeScan<T, R>(project: (acc: R, x: T) => Observable<R>, seed: R).
Would it be better to expose concurrency parameter?