--- classify { |o| ... }
#    Classifies the set by the return value of the given block and
#    returns a hash of {value => set of elements} pairs.  The block is
#    called once for each element of the set, passing the element as
#    parameter.
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    { => Ǥν} Ȥ򤷤ϥåǤ
    ֥åϽΤ٤ƤǤФ1٤ļ¹Ԥ졢
    ֥åѿ o ˤϤǤϤޤ

    :

      require 'set'
      files = Set.new(Dir.glob("*.rb"))
      hash = files.classify { |f| File.mtime(f).year }
      p hash    #=> {2000=>#<Set: {"a.rb", "b.rb"}>,
                #    2001=>#<Set: {"c.rb", "d.rb", "e.rb"}>,
                #    2002=>#<Set: {"f.rb"}>}

