test.rb

module WhatAmIMissing
  module Cat
    module Bill
      def thing
        [1,2,3]
      end
    end
  end

  class John
    include Cat::Bill
    
    def thing
      # ALSO: `concat` rather than `concat!`
      super.concat([5,5])
    end
  end
end

WhatAmIMissing::John.new.thing
=> [1, 2, 3, 5, 5]