conv => 
  rhs
  rw [Nat.add_comm]
y : Packet
xs : List Packet
p : ¬(IsSingle (Multiple xs) ∧ IsSingle y)
⊢ levels (children y) + levels (children (Multiple xs)) < level (Multiple xs) + level y
y : Packet
xs : List Packet
p : ¬(IsSingle (Multiple xs) ∧ IsSingle y)
⊢ levels (children y) + levels (children (Multiple xs)) < level y + level (Multiple xs)