⊃ — Mix / Pick⊃Y assembles items into rectangular cells, padding to a common shape with fill.
⊃1 2 3 ⍝ 1 2 3
⊃(1 2⋄ 3 4 5) ⍝ 2 3⍴1 2 0 3 4 5
P⊃Y selects leading-axis coordinates. Remaining axes form a cell; full coordinates return the stored value. Successive Picks traverse nesting. Empty coordinates return Y.
2⊃2 3⍴⍳6 ⍝ 4 5 6
2 1⊃2 3⍴⍳6 ⍝ 4
1⊃2⊃(1 2⋄ 3 4) ⍝ 3
⍬⊃1 2 ⍝ 1 2
Array-valued coordinate fields pair using leading-axis agreement:
(1 3)˘(2 4)⊃3 4⍴⍳12 ⍝ 2 12
On Mix, axes place cell dimensions among frame dimensions; fractional positions insert before an axis. INDEX: position outside the array. Index 1 on an empty axis gives fill, as does First.