user15543: In his article “The Horse Concoction", Tim Krabbe mentions retrograde analysis problems that involve placing 10 rooks, knights, or bishops on the board for a mate in 1. The 10 rooks problem shown (it also on site).  However, the other two by Henrik Juel and Guus Rol are only mentioned. Who made which is unclear, although suspect that it goes respectively.
Here is the relevant portion of the article.

"In the last few years this old idea, where all the pawns of one or even both sides promote to one kind of piece, had been taken up again by the Dane Henrik Juel and the Dutchman Guus Rol - first with Bishops, then with Knights. And when Goldsteen saw Rol's latest version of an earlier composition by Juel, something unusual happened to him: he was jealous. He was just pissed off that he hadn't thought of Juel's and Rol's ten-knight problem (add ten black Knights to a given position so that White can mate in 1)."

Can these problems be found online? I would love to see them out of curiosity and to learn more about retrograde analysis.

user8201: Henrik Juel's three "add pieces" problems listed above (together with a fourth one) are all to be found, together with detailed solutions in PDB, using the query: a='juel' and K='add pieces'.
Here is the superb fourth one, with its surprising solution...
[Title "Henrik Juel. Probleemblad, 5/1997. Add a unit. What was the last move?"]
[FEN "4k3/ppp1pp2/4pp2/8/3P4/3P2P1/1PpP1PP1/2K5 w - - 0 1"]


15543: <p>In his article <a href="https://timkr.home.xs4all.nl/tour/horse.htm" rel="nofollow noreferrer">“The Horse Concoction&quot;</a>, Tim Krabbe mentions retrograde analysis problems that involve placing 10 rooks, knights, or bishops on the board for a mate in 1. The 10 rooks problem shown (it also <a href="https://chess.stackexchange.com/questions/13952/how-is-this-retrograde-analysis-position-legal">on</a> site).  However, the other two by Henrik Juel and Guus Rol are only mentioned. Who made which is unclear, although suspect that it goes respectively.</p>
<p>Here is the relevant portion of the article.</p>
<blockquote>
<p>&quot;In the last few years this old idea, where all the pawns of one or even both sides promote to one kind of piece, had been taken up again by the Dane Henrik Juel and the Dutchman Guus Rol - first with Bishops, then with Knights. And when Goldsteen saw Rol's latest version of an earlier composition by Juel, something unusual happened to him: he was jealous. He was just pissed off that he hadn't thought of Juel's and Rol's ten-knight problem (add ten black Knights to a given position so that White can mate in 1).&quot;</p>
</blockquote>
<p>Can these problems be found online? I would love to see them out of curiosity and to learn more about retrograde analysis.</p>
 8201: <p>Henrik Juel's three &quot;add pieces&quot; problems listed above (together with a fourth one) are all to be found, together with detailed solutions <a href="https://pdb.dieschwalbe.de/search.jsp?expression=a%3D%27juel%27+AND+K%3D%27Erg%C3%A4nze+Steine%27" rel="nofollow noreferrer">in PDB, using the query: a='juel' and K='add pieces'</a>.</p>
<p>Here is the superb fourth one, with its surprising solution...</p>
<pre><code>[Title &quot;Henrik Juel. Probleemblad, 5/1997. Add a unit. What was the last move?&quot;]
[FEN &quot;4k3/ppp1pp2/4pp2/8/3P4/3P2P1/1PpP1PP1/2K5 w - - 0 1&quot;]
</code></pre>
