Shelf with a dependency-managing wrapper method

shelf.rb

class Shelf # without the wrapper
  ...
  def store(book)
    # imagine complicated logic here...
    intermediate_result = book.storage_requirements / self.capacity
    # imagine more complicated logic...
  end
end

class Shelf # with wrapper
  ...
  def store(book)
    # imagine complicated logic here...
    intermediate_result = storage_requirements(book) / capacity
    # imagine more complicated logic...
  end
  
  def storage_requirements(book)
    book.storage_requirements
  end
end