public static Source instance(Context context) {
        Source instance = context.get(sourceKey);
        if (instance == null) {
            Options options = Options.instance(context);
            String sourceString = options.get("-source");
            if (sourceString != null) instance = lookup(sourceString);
            if (instance == null) instance = DEFAULT;
            context.put(sourceKey, instance);
        }
        return instance;
    }