induction p with
| nil => simp
| cons _ _ ih => simp [ih, add_comm, add_left_comm, add_assoc]
V : Type u
V' : Type v
V'' : Type w
G : Digraph V
G' : Digraph V'
G'' : Digraph V''
u v w : V
p : Walk G u v
q : Walk G v w
⊢ length (append p q) = length p + length q
no goals