∘., ?, is the empty list ⍬ :
⍬,3 2 1,⍬
3 2 1
But that's not the identity element of ∘., :
⍬ ∘., 3 2 1 ⍝ APL keeps not printing anything!
The identity element of an outer product needs to be an array whose shape is the empty list!
⍴⍬
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