Thought experiment: What if i. does overtake on the arguments to ensure that the item shape of the left argument matches the cell shape of the right argument?
I=: 4 : '(s{."c x) i. (s{."c y) [ s=. (}.$x)>.(-c=. <:$$x){.$y'
x=: > ;:'zero one two five one'
y=: > ;:'three one four one five nine'
$x
5 4
$y
6 5
x i. y
5 5 5 5 5 5
x I y
5 1 5 1 3 5
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