# File lib/rspec/core/configuration.rb, line 307
      def mock_with(framework)
        framework_module = case framework
        when Module
          framework
        when String, Symbol
          require case framework.to_s
                  when /rspec/i
                    'rspec/core/mocking/with_rspec'
                  when /mocha/i
                    'rspec/core/mocking/with_mocha'
                  when /rr/i
                    'rspec/core/mocking/with_rr'
                  when /flexmock/i
                    'rspec/core/mocking/with_flexmock'
                  else
                    'rspec/core/mocking/with_absolutely_nothing'
                  end
          RSpec::Core::MockFrameworkAdapter
        end

        new_name, old_name = [framework_module, @mock_framework].map do |mod|
          mod.respond_to?(:framework_name) ?  mod.framework_name : :unnamed
        end

        unless new_name == old_name
          assert_no_example_groups_defined(:mock_framework)
        end

        if block_given?
          raise "#{framework_module} must respond to `configuration` so that mock_with can yield it." unless framework_module.respond_to?(:configuration)
          yield framework_module.configuration
        end

        @mock_framework = framework_module
      end