The answer of exercise 34 on http://reactive-extensions.github.io/learnrx/

learnrx-exercise34.js

function(sprite, spriteContainer) {
    // All of the mouse event sequences look like this:
    // seq([ {pageX: 22, pageY: 3423, offsetX: 14, offsetY: 22} ,,, ])
    var spriteMouseDowns = Observable.fromEvent(sprite, "mousedown"),
        spriteContainerMouseMoves = Observable.fromEvent(spriteContainer, "mousemove"),
        spriteContainerMouseUps = Observable.fromEvent(spriteContainer, "mouseup"),        
        // Create a sequence that looks like this:
        // seq([ {pageX: 22, pageY:4080 },,,{pageX: 24, pageY: 4082},,, ])
        spriteMouseDrags =
            // For every mouse down event on the sprite...
            spriteMouseDowns.
                flatMap(function(contactPoint) {
                    // ...retrieve all the mouse move events on the sprite container...
                    return spriteContainerMouseMoves.map(function(e) {
                        return {
                            pageX : e.pageX - contactPoint.offsetX,
                            pageY : e.pageY - contactPoint.offsetY
                        };
                    });
                }).
                takeUntil(spriteContainerMouseUps);
    
    // For each mouse drag event, move the sprite to the absolute page position.
    spriteMouseDrags.forEach(function(dragPoint) {
        sprite.style.left = dragPoint.pageX + "px";
        sprite.style.top = dragPoint.pageY + "px";
    });
}