cases x
x y : Packet
p : ¬(IsSingle x ∧ IsSingle y)
⊢ levels (children x) + levels (children y) < level x + level y
case Single
y : Packet
a✝ : Nat
p : ¬(IsSingle (Single a✝) ∧ IsSingle y)
⊢ levels (children (Single a✝)) + levels (children y) < level (Single a✝) + level y

case Multiple
y : Packet
a✝ : List Packet
p : ¬(IsSingle (Multiple a✝) ∧ IsSingle y)
⊢ levels (children (Multiple a✝)) + levels (children y) < level (Multiple a✝) + level y