L.mapbox.accessToken = 'pk.eyJ1IjoibWFwdGFzdGlrIiwiYSI6IjNPMkREV1kifQ.2KGPFZD0QaGfvYzXYotTXQ';
var handle = document.getElementById('handle'),
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    startTop;

var map = L.mapbox.map('map', 'maptastik.nc2c5lfc',{
      minZoom: 16,
      maxZoom: 18
    })
    .setView([38.042966,-84.495238], 17);

var overlay = L.tileLayer('http://mapwarper.net/maps/tile/10505/{z}/{x}/{y}.png', {
      zIndex: 2,
      attribution: '<a href="http://libraries.uky.edu/" target="_blank">University of Kentucky Libraries</a>, <a href="http://mapwarper.net/" target="_blank">Map Warper</a>',
      opacity: 0.75
    })
    .addTo(map);

document.onmousemove = function(e) {
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    // Adjust opacity.
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};

handle.onmousedown = function(e) {
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    startTop = handle.offsetTop - 5;
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};

document.onmouseup = function(e) {
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};