Mul ← +˝∘×⎉1‿∞

(>⟨
  ⟨1, 2, 3⟩
  ⟨4, 5, 6⟩
  ⟨7, 8, 9⟩
⟩) Mul >⟨
  ⟨1,  2,  3, 4⟩
  ⟨5,  6,  7, 8⟩
  ⟨9, 10, 11, 12⟩
⟩