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value-struct.rb

  class ImmutabilityError < StandardError; end

  Contract Hash => DescendantOf[KeywordStruct]
  def self.define(**args)
    KeywordStruct.new(*args.keys) do
      include C

      Contract KeywordArgs[**args] => Any
      def initialize(**params)
        super
      end

      args.keys.each do |k|
        define_method "#{k}=" do |_val|
          raise ImmutabilityError.new
        end
      end
    end
  end