Waiting that someone else gives a better answer, you can use this function

(lambda ()(stree->tree '(line (point "0par" "0par") (point "1par" "0par"))))

which you can conveniently put in an extern macro and assign to a
"separator" macro:

<assign|separator|<extern|(lambda ()(stree-\<gtr\>tree '(line (point "0par"
"0par") (point "1par" "0par"))))>>

G.


On 12.02.21 14:53, Frank wrote:
> Dear all,
>
> I wonder how to insert an horizontal separator. For example, when I
> prepare a set of problems, I want to put a separator so that the
> exercises in each group are closely related.
>
> Best,
> Frank