Dependency = function(who, dependency) {
    var v = eval(dependency);
    if (typeof v === "undefined" || v === null) {
        throw new Error("Cannot find class '" + dependency + "' required for " +
                        "'" + who + "'");
    }
}

AbstractMethod = function(who, instance, method) {
    if (typeof instance[method] !== "function") {
        throw new Error("Abstract method '" + method + "' of '" + who + "' is not implemented!")
    }
}