_valid_weeknum?

# File lib/date.rb, line 658
    def _valid_weeknum? (y, w, d, f, sg=GREGORIAN) # :nodoc:
      if d < 0
        d += 7
      end
      if w < 0
        ny, nw, nd, nf =
          jd_to_weeknum(weeknum_to_jd(y + 1, 1, f, f, sg) + w * 7, f, sg)
        return unless ny == y
        w = nw
      end
      jd = weeknum_to_jd(y, w, d, f, sg)
      return unless [y, w, d] == jd_to_weeknum(jd, f, sg)
      jd
    end