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//creates an ordered set, with no duplicate elements
var orderedset = function (elements) {
    for(i=0;i<elements.length;i++) this[elements[i]] = i;
};
//gets an item from the ordered set, throws an index error if i not in order.
function getitem(ordset, i) {
    for(var elem in ordset) if(ordset[elem] == i) return elem;
    throw "Index error";
}
var f = new orderedset(["a", "b", "c"]);
console.log(getitem(f, 1));
// b