function chain() {
    for(var index = 0; index < arguments.length; ++index) {
        try {
            var result = arguments[index]();
            return result;
        } catch(ignore) {}
    }
}

function make_worker() {
    var result = chain(
        function() {return new ActiveXObject('Msxml2.XMLHTTP');},
        function() {return new ActiveXObject('Microsoft.XMLHTTP');},
        function() {return new XMLHttpRequest();}
    );
    return result;
}