2. Index-Of Selfie Index   <<   >>
 

x⍳x

((⍳≢x)=x⍳x)⌿x   nub (unique)
(x⍳x)∘.=(x⍳y) an efficient computation for x∧.=⍉y for matrices x and y
(x⍳x)∘.=(x⍳y) an efficient computation for x∘.≡y for non-simple vector x
(x⍳x) f⌸ y equivalent to x f⌸ y