↑ — Mix / Take↑Y assembles nested items into cells, padding to a common shape.
↑(1 2)(3 4 5) ⍝ 2 3⍴1 2 0 3 4 5
N↑Y takes leading items; negative N takes from the end. Overtake inserts fill.
4↑1 2 ⍝ 1 2 0 0
¯4↑1 2 ⍝ 0 0 1 2
Vector counts apply to successive leading axes. [K] selects axes.
1 2↑2 3⍴⍳6 ⍝ 1 2⍴1 2
On Mix, axes place cell dimensions among frame dimensions; fractional positions insert before an axis.