public Repl(Class<? extends Visitor<S, T>> vClass) {
        this.interp = null;
        evalClass = vClass;
        try {
            interp = evalClass.newInstance();
        } catch (InstantiationException | IllegalAccessException ie) {
            System.err.println(ie.getMessage());
            System.err.println("Fatal error: Failed to instantiate "
                    + "interpreter!  Terminating...");
            System.exit(1);
        }
    }