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ko.extenders.sumColumn = function (target, options) {
    var result = ko.computed({
        read: target,
        write: function () {
            var sum = 0, dec = 2;
            if (!isNaN(options.decimal))
                dec = options.decimal;
            var keys = Object.getOwnPropertyNames(options.summary);
            var i = keys.indexOf(options.name);
            $.each(options.array(), function () {
                sum += +this[keys[i]]();
            });
            target(sum.toFixed(dec));
        }
    }).extend({ notify: "always" });
    result(target());
    return result;
};