exact step'
case a
α : Type
inst✝ : DecidableEq α
start : List (α × Bool)
letter : α × Bool
nilP : ¬start = []
a : α
b : Bool
cancP : List.getLast start nilP = (a, b)⁻¹
sp : List.dropLast start ++ [((a, b).1, !(a, b).2)] = start
step' : Step (start ++ [(a, b)]) (List.dropLast start)
this : List.dropLast start ++ [(a, !b), (a, b)] = List.dropLast start ++ [(a, !b)] ++ [(a, b)]
⊢ Step (start ++ [(a, b)]) (List.dropLast start)
no goals