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: options.rb
    path of file: ./repos/concurrent-ruby/lib/concurrent-ruby/concurrent
    
    the code of the file until where you have to start completion: require 'concurrent/configuration'

module Concurrent

  # @!visibility private
  module Options

    # Get the requested `Executor` based on the values set in the options hash.
    #
    # @param [Hash] opts the options defining the requested executor
    # @option opts [Executor] :execut
    
    def self.executor(executor_identifier)
      case executor_identifier
      when :fast
        Concurrent.global_fast_executor
      when :io
        Concurrent.global_io_executor
      when :immediate
        Concurrent.global_immediate_executor
      when Concurrent::ExecutorService
        executor_identifier
      else
        raise ArgumentError, "executor not recognized by '#{executor_identifier}'"
      end