˝)𝔽˝ 𝕩: InsertFold 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 ⟨ 9 12 15 ⟩ 0‿1‿2 + 3‿4‿5 + 6‿7‿8 ⟨ 9 12 15 ⟩
𝕨 𝔽˝ 𝕩: Insert With InitialMonadic insert, but use 𝕨 as initial right argument.
b ← 3‿3 ⥊ ↕9 1‿1‿1 +˝ b ⟨ 10 13 16 ⟩ 1 +˝ b ⟨ 10 13 16 ⟩ 0‿1‿2 + 3‿4‿5 + 6‿7‿8 + 1‿1‿1 ⟨ 10 13 16 ⟩