------------------------------------------------------------ Class: Hash
      A  Hash  is a collection of key-value pairs. It is similar to an
       Array , except that indexing is done via arbitrary keys of any
      object type, not an integer index. The order in which you traverse
      a hash by either key or value may seem arbitrary, and will
      generally not be in the insertion order.
      Hashes have a _default value_ that is returned when accessing keys
      that do not exist in the hash. By default, that value is  nil .
      Enumerable(all?, any?, collect, detect, each_cons, each_slice,
      each_with_index, entries, enum_cons, enum_slice, enum_with_index,
      find, find_all, grep, include?, inject, map, max, member?, min,
      partition, reject, select, sort, sort_by, to_a, to_set, zip)
 Class methods:
      [], new
 Instance methods:
      ==, , =, clear, default, default=, default_proc, delete,
      delete_if, each, each_key, each_pair, each_value, empty?, fetch,
      has_key?, has_value?, include?, index, indexes, indices,
      initialize_copy, inspect, invert, key?, keys, length, member?,
      merge, merge!, rehash, reject, reject!, replace, select, shift,
      size, sort, store, to_a, to_hash, to_s, update, value?, values,


 ==== Hash ====
   {a=>b, ... }
 Object#hash [Object/hash] メソッドが使われ、キーの同一性判定には、
 Object#eql? [Object/eql?] メソッドが使われます。
 キーとして与えたオブジェクトの内容が変化し、メソッド hash の返す
 値が変わるとハッシュから値が取り出せなくなりますから、ArrayHash などのインスタンスはキーに向きません。文字列をキーとして与
 TypeError が発生します。
 ---- Singleton methods ----
 [] new
 ---- Instance methods ----
  = clear clone default default= default_proc delete
 delete_if dup each each_key each_pair each_value empty?
 fetch has_key? has_value? include? index indexes indices
 invert key? keys length member? merge merge! rehash reject
 reject! replace shift size store to_a to_hash update value?
 values values_at
 ---- Singleton methods (inherited) ----
 ---- Instance methods (inherited) ----
 == === =~ __id__ __send__ _dump _load all? any? class
 collect detect display each_with_index entries eql? equal?
 extend find find_all freeze frozen? grep hash id initialize
 initialize_copy inject inspect instance_eval instance_of?
 instance_variable_get instance_variable_set
 instance_variables is_a? kind_of? map marshal_dump
 marshal_load max method method_missing methods min nil?
 object_id partition pretty_print pretty_print_cycle
 pretty_print_instance_variables private_methods
 protected_methods public_methods remove_instance_variable
 respond_to? select send singleton_method_added
 singleton_method_removed singleton_method_undefined
 singleton_methods sort sort_by taint tainted? to_ary to_int
 to_s to_str type untaint zip