How to replace bracket-axis with rank¶

MARPLE does not have bracket-axis notation (f[k]). Use the rank operator instead:

Traditional bracket-axis	Rank equivalent	Meaning
`+/V`	`+/V`	Reduce along last axis (same)
`+/[1]M`	`(+/⍤¯1) M`	Reduce along first axis
`⌽[1]M`	`(⌽⍤¯1) M`	Reverse along first axis

Negative rank means "complementary rank" -- the cell rank is computed as array_rank + k. So ⍤¯1 on a matrix (rank 2) gives cell rank 1, which means the frame is the first axis.

For user-defined functions, rank is the only option:

      myFn ← {⍵[⍋⍵]}
      (myFn⍤1) M

There is no bracket-axis equivalent for user dfns.