Origins of the fold-in

On the poet Abu 'l-‘Ibar al-Hashimi (d. 866 CE), by Sinan Antoon,
The Poetics of the Obscene in Premodern Arabic Poetry: Ibn al-Hajjaj
and Sukhf (Palgrave Macmillan, 2014), 40:

¹⁸⁹  From the Book of Songs

From the Epistle of the Two Luminaries

In his Epistle of the Two Luminaries, which is [subtitled] "From a dejected lover, to one whose love is reciprocated by another," and begins with the words: "The earth lies before the merciful king, the sultan of beauty, the lion of combat...." ‘Ala’ al-Din [Taqi al-Din] ibn al-Maghribi said (meter: majzū’ al-ramal):

   The Nile comes and goes.
       My love goes on and on.
   Nothing I say tomorrow will be enough.
       Sometimes love is too much.
   Every heart but mine
       gets the love it wants.
   I am the lone unfortunate
       going steady with rejection.

Then ‘Ala’ al-Din [Taqi al-Din] said: I am the lone unfortunate who pissed on a plate of fried doughnuts, dribbling out a vinegar stream. I crucified Iblis with his own hammer, and left him sagging and singing "Tra-la-la-la!" as he flapped his wings like a chicken (meter: majzū’ al-ramal):

   Tra-la-la-la, tra-la-la!
       You, with the eyes of a little gazelle!
   God have mercy on my slayer.
       Me it is no boast to kill.

From The Register of Ardent Love by Ibn Abi Hajala