Challenge fenced div with no inner fenced divs is converted to Discussion

[alex-ball]

Challenge fenced divs display as intended when they contain another fenced div, such as a Hint or Solution:

::::::::::::::::::::::::::::::::: challenge

### CHALLENGE

I look like a CHALLENGE.

::::::::::::::::: solution

### SOLUTION

I look like a SOLUTION.

::::::::::::::::::::::::::

:::::::::::::::::::::::::::::::::::::::::::

But if they do not contain another fenced div, they are rendered as a Discussion:

::::::::::::::::: discussion

### DISCUSSION

I really am a DISCUSSION.

::::::::::::::::::::::::::::

:::::::::::::::::: challenge

### CHALLENGE

But I look like a DISCUSSION.

::::::::::::::::::::::::::::

As far as I can tell, this is because of the challenge_block function in lesson.lua, which contains the following lines:

sandpaper/inst/rmarkdown/lua/lesson.lua

Lines 578 to 581 in 84f602b

	if #this_challenge.content > 1 then
	bookend = pandoc.Div(this_challenge.content, {class = "discussion"})
	challenge_train:insert(callout_block(bookend))
	end

Here, the challenge content is put in a div with the class "discussion". On the principle of least surprise, I would have thought the class should be "challenge".

Apologies if this is intended behaviour and I missed the documentation for it somewhere.