∇∇
Inside a dop, ∇∇ denotes the operator itself. Apply operands again to derive the recursive function; ∇ instead retains the current operands.
∇
op←{⍵=0:0 ⋄ 1+(⍺⍺ ∇∇)⍵-1} ⋄ (+op)3 ⍝ 3