IMPORTANT: Please do not post solutions, hints, or other spoilers
        until at least 60 hours after the date of this message.
        Thanks.

IMPORTANTE: Por favor, no enviéis soluciones, pistas, o cualquier otra
        cosa que pueda echar a perder la resolución del problema hasta
        que hayan pasado por lo menos 60 horas desde el envío de este
        mensaje. Gracias.

IMPORTANT: S'il vous plaît, attendez au minimum 60 heures après la
        date de ce message avant de poster solutions, indices ou autres
        révélations. Merci.

WICHTIG: Bitte schicken Sie keine Lösungen, Tipps oder Hinweise für
        diese Aufgabe vor Ablauf von 60 Stunden nach dem Datum dieser
        Mail. Danke.

Qing3 Zhu4Yi4: Qing3 Ning2 Deng3Dao4 Jie1Dao4 Ben3 Xin4Xi2 Zhi1Hou4 60
        Xiao3Shi2, Zai4 Fa1Biao3 Jie3Da2, Ti2Shi4, Huo4 Qi2Ta1 Hui4
        Xie4Lou4 Da2An4 De5 Jian4Yi4.  Xie4Xie4.

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Write a function, format_number_list, whose argument is a list of
integers.  It then returns a string which represents the input list in
compact, human-readable form.

For example,

         format_number_list(1, 2, 4, 5, 6, 7, 9, 13, 24, 25, 26, 27)

will return

        "1-2, 4-7, 9, 13, 24-27"

Also write a function, 'expand_number_list', which does the conversion
in the opposite direction, so that

        expand_number_list("1-2, 4-7, 9, 13, 24-27")

will return

        (1, 2, 4, 5, 6, 7, 9, 13, 24, 25, 26, 27)


(Thanks to Mark Hickman for suggesting this quiz problem.)