abc←{{0=⍴⍵:1 ⋄ 0=⍴h←⊃⍵:0 ⋄ ∇(t←1↓⍵)~¨⊃h:1 ⋄ ∇(⊂1↓h),t}⍸¨↓⍵∘.∊⍺}