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From Rank & Friends, Dyalog ’13.

inverted-table index-of   {(⍉↑⍺⍳¨⍺)⍳(⍉↑⍺⍳¨⍵)}
implemented in v14.0 as   8⌶
   x ← x0 x1 x2 x3 …
   y ← y0 y1 y2 y3 …
   i ← x (8⌶) y

8⌶ exploits:
•  CRC32 instruction, SSE4.2, circa 2008
•  “selfies”, x(8⌶)x
•  other black arts ☺