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<script>
headings = [];
onload = function(){
headings = document.querySelectorAll('h2');
};
onscroll = function(e){
var heading = find(window.scrollY);
if (!heading) return;
var links = document.querySelectorAll('#menu a')
, link;
for (var i = 0, len = links.length; i < len; ++i) {
link = links[i];
link.className = link.getAttribute('href') == '#' + heading.id
? 'active'
: '';
}
};
function find(y) {
var i = headings.length
, heading;
while (i--) {
heading = headings[i];
if (y >= heading.offsetTop) {
return heading;
}
}
}
</script>
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