apply False.elim
x y : Nat
p : ¬(IsSingle (Single x) ∧ IsSingle (Single y))
⊢ levels (children (Single x)) + levels (children (Single y)) < level (Single x) + level (Single y)
case h
x y : Nat
p : ¬(IsSingle (Single x) ∧ IsSingle (Single y))
⊢ False