slightly more compact data;

--- a/src/Pure/General/set.ML	Tue Mar 28 23:16:27 2023 +0200
+++ b/src/Pure/General/set.ML	Wed Mar 29 10:34:50 2023 +0200
@@ -50,6 +50,8 @@
   Branch3 of T * elem * T * elem * T;
 
 fun make2 (Empty, e, Empty) = Leaf1 e
+  | make2 (Leaf1 e1, e2, Empty) = Leaf2 (e1, e2)
+  | make2 (Empty, e1, Leaf1 e2) = Leaf2 (e1, e2)
   | make2 arg = Branch2 arg;
 
 fun make3 (Empty, e1, Empty, e2, Empty) = Leaf2 (e1, e2)

--- a/src/Pure/General/table.ML	Tue Mar 28 23:16:27 2023 +0200
+++ b/src/Pure/General/table.ML	Wed Mar 29 10:34:50 2023 +0200
@@ -87,6 +87,8 @@
   Branch3 of 'a table * (key * 'a) * 'a table * (key * 'a) * 'a table;
 
 fun make2 (Empty, e, Empty) = Leaf1 e
+  | make2 (Leaf1 e1, e2, Empty) = Leaf2 (e1, e2)
+  | make2 (Empty, e1, Leaf1 e2) = Leaf2 (e1, e2)
   | make2 arg = Branch2 arg;
 
 fun make3 (Empty, e1, Empty, e2, Empty) = Leaf2 (e1, e2)