private IObservable<SelectionViewModel> GetSelection(UIElement relativeTo)
{
IObservable<SelectionViewModel> mouseMoveObservable =
from mouseDown in relativeTo.GetMouseDown()
let mouseDownPosition = mouseDown.EventArgs.GetPosition(relativeTo)
from mouseMove in relativeTo.GetMouseMove().TakeUntil(relativeTo.GetMouseUp())
let mouseMovePosition = mouseMove.EventArgs.GetPosition(relativeTo)
select new SelectionViewModel(mouseDownPosition, mouseMovePosition);
return mouseMoveObservable;
}