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Fold over cells of 𝕩 with 𝔽 from end to start, that is, insert 𝔽 between the major cells of 𝕩.
a ← 3‿3 ⥊ ↕9
+˝ a
0‿1‿2 + 3‿4‿5 + 6‿7‿8
Monadic insert, but use 𝕨 as initial right argument.
b ← 3‿3 ⥊ ↕9
1‿1‿1 +˝ b
1 +˝ b
0‿1‿2 + 3‿4‿5 + 6‿7‿8 + 1‿1‿1