have : L₁ ++ [(x, b), (x, !b)] = 
  (L₁ ++ [(x, b)]).concat (x, !b) := by simp
α : Type
inst✝ : DecidableEq α
start : List (α × Bool)
letter : α × Bool
reduced_start : Reduced start
nilP : ¬start = []
L' : List (α × Bool)
step : Step (start :+ letter) L'
x : α
b : Bool
L₁ L₂ : List (α × Bool)
eqs : start :+ letter = L₁ ++ (x, b) :: (x, !b) :: L₂ ∧ L' = L₁ ++ L₂
cancP : ¬List.getLast start nilP = letter⁻¹
endP : L₂ = []
eq1 : List.concat start letter = L₁ ++ [(x, b), (x, !b)]
⊢ False
α : Type
inst✝ : DecidableEq α
start : List (α × Bool)
letter : α × Bool
reduced_start : Reduced start
nilP : ¬start = []
L' : List (α × Bool)
step : Step (start :+ letter) L'
x : α
b : Bool
L₁ L₂ : List (α × Bool)
eqs : start :+ letter = L₁ ++ (x, b) :: (x, !b) :: L₂ ∧ L' = L₁ ++ L₂
cancP : ¬List.getLast start nilP = letter⁻¹
endP : L₂ = []
eq1 : List.concat start letter = L₁ ++ [(x, b), (x, !b)]
this : L₁ ++ [(x, b), (x, !b)] = List.concat (L₁ ++ [(x, b)]) (x, !b)
⊢ False