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Returns the week of the year according to the ISO 8601 specification. The
spec defines the first week of the year as the week which contains at
least 4 days of the new year. The week starts at Monday. Therefore
December 29th - 31th could belong to the next year and January 1st - 3th
could belong to the previous year. If January 1st is on Thursday (or
earlier) it belongs to the first week, otherwise to the last week of the
previous year. Hence January 4th always belongs to the first week while
the December 28th always belongs to the last week.