simp only [List.length_pmap, beq_eq_false_iff_ne, ne_eq]
m n : Nat
i : Fin m
j : Fin n
⊢ (List.length
          (List.pmap Fin.restrict (Board.path i j)
            (_ : ∀ (a : Fin ((m + 1) * (n + 1))), a ∈ Board.path i j → ↑a < (m + 1) * (n + 1) - 1)) %
        2 ==
      1) =
    false
m n : Nat
i : Fin m
j : Fin n
⊢ ¬List.length (Board.path i j) % 2 = 1