```ruby
module Nestable

  def nests(mapping)
    # Setup an accessor for all nested instances.
    attr_accessor *mapping.keys
    # Create a nested instance automatically on initialize.
    self.define_method(:initialize) do |arguments = nil|
      mapping.each do |attribute, klass|
        self.instance_variable_set "@#{attribute}".to_sym, klass.new
      end
      super arguments if arguments
    end

  end

end
```

Call private method define_method correctly.
user
```ruby
module Nestable

  def nests(mapping)
    # Setup an accessor for all nested instances.
    attr_accessor *mapping.keys
    # Create a nested instance automatically on initialize.
    define_method(:initialize) do |arguments = nil|
      mapping.each do |attribute, klass|
        self.instance_variable_set "@#{attribute}".to_sym, klass.new
      end
      super arguments if arguments
    end

  end

end
```
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