```ruby
module Mozart
  def self.value(*field_names, &block)
    Class.new do
      define_method(:initialize) do |params={}|
        raise ArgumentError unless params.keys == field_names

        self.data = {}

        params.each { |k,v| data[k] = v.freeze }

        data.freeze
      end

      def ==(other)
        self.class == other.class && data == other.data
      end

      alias_method :eql?, :==

      def hash
        data.hash
      end

      field_names.each do |name|
        define_method(name) { data[name] }
      end

      protected

      attr_accessor :data
    end
  end
end
```

Use a less clashy name
user
```ruby
module Mozart
  def self.value(*field_names, &block)
    Class.new do
      define_method(:initialize) do |params={}|
        raise ArgumentError unless params.keys == field_names

        self.__data__ = {}

        params.each { |k,v| __data__[k] = v.freeze }

        __data__.freeze
      end

      def ==(other)
        self.class == other.class && data == other.data
      end

      alias_method :eql?, :==

      def hash
        __data__.hash
      end

      field_names.each do |name|
        define_method(name) { __data__[name] }
      end

      protected

      attr_accessor :__data__
    end
  end
end
```
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