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↑ 𝕩: Prefixes
Prefixes of array 𝕩 along its first axis.
↑ 1‿2‿3‿4
a ← 3‿3 ⥊ ↕9
↑ a
𝕨 ↑ 𝕩: Take
For each integer in 𝕨, take that many elements from each dimension of 𝕩.
Negative numbers take from the end.
If any of the elements in 𝕨 are greater than the length of their respective dimension, the dimension is extended with a fill value.
3 ↑ 1‿3‿5‿67
b ← 4‿4 ⥊ ↕16
3‿3 ↑ b
5‿5 ↑ b
3‿¯3 ↑ b