You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: backend.rb
    path of file: ./repos/mobility/lib/mobility
    
    the code of the file until where you have to start completion: # frozen-string-literal: true
module Mobility
=begin

Defines a minimum set of shared components included in any backend. These are:

- a reader returning the +model+ on which the backend is defined ({#model})
- a reader returning the +attribute+ for which t
    
      def setup &block
        if @setup_block
          setup_block = @setup_block
          exec_setup_block = method(:exec_setup_block)
          @setup_block = lambda do |attributes, options, backend_class|
            [setup_block, block].each do |blk|
              exec_setup_block.call(self, attributes, options, backend_class, &blk)
            end
          end
        else
          @setup_block = block
        end