@mbaas2 reports via email:

“in the explanation of rules for computation of haystack⍳needles, you wrote

"- items which do not appear in haystack give the result 1+m if m is the leading number in the shape of haystack, i.e. m ← ≢haystack."

I find that confusing, because you speak of "shape" and then use ≢ - and that will never return more than 1 number, so the special emphasis on a leading number provoques unneccessary questions, I think ;)

I'd like to suggest an alternative:

"- items which do not appear in haystack give the result 1+m if m is the number of element of haystack, i.e. m ← ≢haystack."