Write a 1-line model of x⍳y as extended in Dyalog v14.0.

   I ← {+⌿∧⍀(⊂⍤r⊢⍺)∘.≢⊂⍤r⊢⍵ ⊣ r←¯1+⍴⍴⍺}

   ⎕←a←6 5⍴'CarloJohn John DianeRogerLisa '
Carlo
John 
John 
Diane
Roger
Lisa 
   a I 'Lisa '
5
   a I 2 5⍴'Lisa John '
5 1
   I⍨ a
0 1 1 3 4 5