The function Enclose (⊂) creates an array with empty shape, or scalar, whose only element is the given argument:

      ⊂⍬
┌┐
││
└┘
      ⍴⊂⍬

      ((⍳2 3) ∘., (⊂⍬) ∘., (⍳4 5))  ≡  (⍳2 3) ∘., (⍳4 5)
1

So ⊂⍬ is the identity element for ∘., !