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// rollover function
function rollover() { 
    // Loop through all images, looking for the data-rollover attribute
    for(var i = 0; i < document.images.length; i++) {
        var img = document.images[i]; 
        var rollover = img.getAttribute("data-rollover"); 
        if (!rollover) continue;  // Skip images without data-rollover

        // Ensure that the rollover image is in the cache
        (new Image()).src = rollover;

        // Define an attribute to remember the default image URL
        img.setAttribute("data-rollout", img.src);

        // Register the event handlers that create the rollover effect
        img.onmouseover = function() {
            this.src = this.getAttribute("data-rollover");
        };
        img.onmouseout = function() {
            this.src = this.getAttribute("data-rollout");
        };
    }
}