If you go to part 13 of the Lab "A J Introduction", you see this:
<quote>
── ((1) 13 of 16) Permutations ──────────────────────────────
The dyadic function x{y indexes y by x If p is a permutation
p, then p{y permutes y by p.
)
</quote>
However, I think there should be a period between "x" and "If"
Also, I find this description confusing. What does it mean to index an
array? Are y and x both arrays?
Is there some tagging of the array created by the
verb ?. such that J knows the array is a permutation and not just an array?
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