FAQ translation of RxJS' withLatestFrom not preserving semantics

The following code

import Rx from 'rxjs/Rx';

const s1 = Rx.Observable.interval(1000).take(3);
const s2 = Rx.Observable.interval(100).take(30);
const s3 = s1.withLatestFrom(s2, (v1,v2) => [v1,v2])
s3.subscribe(function (x) { console.log(x) })

outputs

[ 0, 8 ]
[ 1, 18 ]
[ 2, 28 ]

According to FAQ, withLatestFrom can be expressed in xstream as follows:

import xs from 'xstream'

const s1 = xs.periodic(1000).take(3);
const s2 = xs.periodic(100).take(30);
const s3 = s1.map((v1) => s2.map((v2) => [v1,v2])).flatten(); 
s3.addListener({
    next: x => console.log(x),
    error: err => console.error(err),
    complete: () => console.log('completed')
})

However, it outputs 30 tuples instead of 3:

[ 0, 0 ]
[ 0, 1 ]
[ 0, 2 ]
[ 0, 3 ]
[ 0, 4 ]
[ 0, 5 ]
[ 0, 6 ]
[ 0, 7 ]
[ 0, 8 ]
[ 1, 9 ]
[ 1, 10 ]
[ 1, 11 ]
[ 1, 12 ]
[ 1, 13 ]
[ 1, 14 ]
[ 1, 15 ]
[ 1, 16 ]
[ 1, 17 ]
[ 1, 18 ]
[ 2, 19 ]
[ 2, 20 ]
[ 2, 21 ]
[ 2, 22 ]
[ 2, 23 ]
[ 2, 24 ]
[ 2, 25 ]
[ 2, 26 ]
[ 2, 27 ]
[ 2, 28 ]
[ 2, 29 ]
completed

[wclr]

I think withLastestFrom should be properly implemented as extra operator

[staltz]

Issue #102 is for implementing withLatestFrom as an extra operator, so I think we can close this one.