n←⟨⟨1⟩,⟨2,4⟩,⟨6,4,9⟩,⟨5,1,7,2⟩⟩
⌊´+´¨((∧´1≥·-˜´˘2↕⊢)¨⊸/∘(⥊·↕≠¨)⊑¨¨<∘⊢)n