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Remove the first element of 𝕩, add a cell of fill values to the end of the first axis of 𝕩.
« 1‿2‿3
« 3‿3 ⥊ 9
Remove the first ≠𝕨 (length) major cells from 𝕩, join 𝕨 to the end of 𝕩. Ranks must match.
78 « 1‿2‿3
8‿5 « 1‿2‿3
a ← 3‿3 ⥊ 9
1‿2‿3 « a