PrettyPrint#group_sub

# File tmp/rubies/ruby-4.1.0-preview1/lib/prettyprint.rb, line 263
def group_sub
  group = Group.new(@group_stack.last.depth + 1)
  @group_stack.push group
  @group_queue.enq group
  begin
    yield
  ensure
    @group_stack.pop
    if group.breakables.empty?
      @group_queue.delete group
    end
  end
end