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Remove the last element of 𝕩, add a cell of fill values to the start of the first axis of 𝕩.
» 1‿2‿3
» 3‿3 ⥊ 9
Remove the last ≠𝕨 (length) major cells from 𝕩, join 𝕨 to the start of 𝕩. Ranks must match.
78 » 1‿2‿3
1‿2 » 1‿2‿3
a ← 3‿3 ⥊ 9
1‿2‿3 » a