def list_available_rewriters
      if Core::Rewriter.availables.empty?
        puts 'There is no snippet under ~/.synvert, please run `synvert --sync` to fetch snippets.'
      else
        Core::Rewriter.availables.each do |group, rewriters|
          puts group
          rewriters.each do |name, rewriter|
            puts '    ' + name
          end
        end
        puts
      end
    end
def list_available_rewriters
      if Core::Rewriter.availables.empty?
        puts 'There is no snippet under ~/.synvert, please run `synvert --sync` to fetch snippets.'
      else
        Core::Rewriter.availables.each do |group, rewriters|
          puts group
          rewriters.each do |name, rewriter|
            puts '    ' + name
          end
        end
        puts
      end
    end
List and print all available rewriters.