Overlay opacity Slider

What would be great would be to have an overlay opacity slider so that the user can change the opacity between the base map and an overlay / maptype / KML..

See here.

www.logic.co/maps/1875-static/

There is slider in the top right corner which is generated from maptilercdn.s3.amazonaws.com/klokantech.js

Written by Petr Klokan at Maptiler

Regards,

Dave

Sorry for double post in advance. Had posted this in wrong section.