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: base.rb
    path of file: ./repos/chef/lib/chef/secret_fetcher
    
    the code of the file until where you have to start completion: #
# Author:: Marc Paradise (<marc@chef.io>)
# Copyright:: Copyright (c) Chef Software Inc.
# License:: Apache License, Version 2.0
#
# Licensed under the Apache License, Version 2.0
    
      def validate!; end

      # Called to fetch the secret identified by 'identifier'.  Implementations
      # should expect that `validate!` has been invoked before `do_fetch`.
      #
      # @param identifier [Object] Unique identifier of the secret to be retrieved.
      # When invoked via DSL, this is pre-verified to be not nil/not empty string.
      # The expected data type and form can vary by implementation.
      # @param version [Object] Optional version of the secret to be retrieved.  If not
      # provided, implementations are expected to fetch the most recent version of the
      # secret by default.
      #
      # @return [Object] The secret as returned from the implementation.  The data type
      # will vary implementation.
      #
      # @raise [Chef::Exceptions::Secret::FetchFailed] if the secret could not be fetched
      def do_fetch(identifier, version); raise NotImplementedError.new; end
    end
  end