def opts(**kws)
      res = {}
      kws.each_pair do |kw, v|
        res[kw] = v[0] unless v[0] == v[1]
      end
      res
    end
def opts(**kws)
      res = {}
      kws.each_pair do |kw, v|
        res[kw] = v[0] unless v[0] == v[1]
      end
      res
    end
strip default-valued options. the value of each kw should be a pair:
  [0] is present value
  [1] is default value