zoom_in = function(x,y)
	-- precondition: Zoomed_out
	-- precondition: Viewport == global_viewport
	-- precondition: Save_viewport exists
	-- center Viewport on x,y at zoom level Saved_viewport.zoom
	animate_viewport_to{
		x=sx(x)-App.screen.width/Save_viewport.zoom/2,
		y=sy(y)-App.screen.height/Save_viewport.zoom/2,
		zoom=Save_viewport.zoom,
	}
	print('Viewport at end of zoom in animation', Viewport.x, Viewport.y, Viewport.zoom)
	Zoomed_out = nil
	Save_viewport = nil	
end